diff --git a/Dockerfile b/Dockerfile
index 4003a01..d17d3db 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -23,4 +23,7 @@ ENV NODE_ENV production
 
 ENV NEXT_TELEMETRY_DISABLED 1
 
-CMD NEXT_PUBLIC_SITE_DOMAIN=$site_domain DEFAULT_DARK_THEME=$dark_theme yarn build && yarn start
+CMD NEXT_PUBLIC_SITE_DOMAIN=$site_domain DEFAULT_DARK_THEME=$dark_theme \
+    NEXT_PUBLIC_DEFAULT_SOURCE_LANG=$default_source_lang \
+    NEXT_PUBLIC_DEFAULT_TARGET_LANG=$default_target_lang \
+    yarn build && yarn start
diff --git a/README.md b/README.md
index 4d6bd06..68d5003 100644
--- a/README.md
+++ b/README.md
@@ -53,6 +53,8 @@ services:
     environment:
       - site_domain=lingva.ml
       - dark_theme=false
+      - default_source_lang=auto
+      - default_target_lang=en
     ports:
       - 3000:3000
 ```
@@ -60,7 +62,7 @@ services:
 #### Docker Run
 
 ```bash
-docker run -p 3000:3000 -e site_domain=lingva.ml -e dark_theme=false thedaviddelta/lingva-translate:latest
+docker run -p 3000:3000 -e site_domain=lingva.ml -e dark_theme=false -e default_source_lang=auto -e default_target_lang=en thedaviddelta/lingva-translate:latest
 ```
 
 ### Vercel
diff --git a/pages/[[...slug]].tsx b/pages/[[...slug]].tsx
index 2c3d459..7824b6a 100644
--- a/pages/[[...slug]].tsx
+++ b/pages/[[...slug]].tsx
@@ -9,7 +9,7 @@ import { useHotkeys } from "react-hotkeys-hook";
 import { CustomHead, LangSelect, TranslationArea } from "@components";
 import { useToastOnLoad } from "@hooks";
 import { googleScrape, extractSlug, textToSpeechScrape } from "@utils/translate";
-import { retrieveFromType, replaceBoth } from "@utils/language";
+import { retrieveFromType, replaceBoth, isValid } from "@utils/language";
 import langReducer, { Actions, initialState } from "@utils/reducer";
 import { localGetItem, localSetItem } from "@utils/storage";
 
@@ -46,18 +46,22 @@ const Page: FC<InferGetStaticPropsType<typeof getStaticProps>> = ({ home, transl
     }, [isLoading, source, target, home, initial]);
 
     useEffect(() => {
-        if (home)
+        if (home) {
+            const localSource = localGetItem("source");
+            const localTarget = localGetItem("target");
             return dispatch({
                 type: Actions.SET_ALL,
                 payload: {
                     state: {
                         ...initialState,
-                        source: localGetItem("source") || initialState.source,
-                        target: localGetItem("target") || initialState.target,
+                        source: isValid(localSource) ? localSource : initialState.source,
+                        target: isValid(localTarget) ? localTarget : initialState.target,
                         isLoading: false
                     }
                 }
             });
+        }
+
         if (!initial)
             return;
 
@@ -218,6 +222,11 @@ export const getStaticProps: GetStaticProps = async ({ params }) => {
             }
         }
 
+    if (!isValid(source) || !isValid(target))
+        return {
+            notFound: true
+        };
+
     const textScrape = await googleScrape(source, target, query);
 
     const [sourceAudio, targetAudio] = await Promise.all([
diff --git a/pages/api/graphql.ts b/pages/api/graphql.ts
index 0161608..79fb703 100644
--- a/pages/api/graphql.ts
+++ b/pages/api/graphql.ts
@@ -2,7 +2,7 @@ import { ApolloServer, gql, IResolvers, ApolloError, UserInputError } from "apol
 import { NextApiHandler } from "next";
 import NextCors from "nextjs-cors";
 import { googleScrape, textToSpeechScrape } from "@utils/translate";
-import { retrieveFromType, getName } from "@utils/language";
+import { retrieveFromType, getName, isValid } from "@utils/language";
 
 export const typeDefs = gql`
     enum LangType {
@@ -33,6 +33,10 @@ export const resolvers: IResolvers = {
     Query: {
         translation(_, args) {
             const { source, target, query } = args;
+
+            if (!isValid(source) || !isValid(target))
+                throw new UserInputError("Invalid language code");
+
             return {
                 source: {
                     lang: {
@@ -49,6 +53,10 @@ export const resolvers: IResolvers = {
         },
         audio(_, args) {
             const { lang, query } = args;
+
+            if (!isValid(lang))
+                throw new UserInputError("Invalid language code");
+
             return {
                 lang: {
                     code: lang
@@ -87,13 +95,7 @@ export const resolvers: IResolvers = {
     Language: {
         name(parent) {
             const { code, name } = parent;
-            if (name)
-                return name;
-
-            const newName = getName(code);
-            if (!newName)
-                throw new UserInputError("Invalid language code");
-            return newName;
+            return name || getName(code);
         }
     }
 };
diff --git a/pages/api/v1/[[...slug]].ts b/pages/api/v1/[[...slug]].ts
index a9fc236..96b4dc5 100644
--- a/pages/api/v1/[[...slug]].ts
+++ b/pages/api/v1/[[...slug]].ts
@@ -1,6 +1,7 @@
 import { NextApiHandler } from "next";
 import NextCors from "nextjs-cors";
 import { googleScrape, textToSpeechScrape } from "@utils/translate";
+import { isValid } from "@utils/language";
 
 type Data = {
     translation: string,
@@ -34,6 +35,9 @@ const handler: NextApiHandler<Data> = async (req, res) => {
 
     const [source, target, query] = slug;
 
+    if (!isValid(target))
+        return res.status(400).json({ error: "Invalid target language" });
+
     if (source === "audio") {
         const audio = await textToSpeechScrape(target, query);
         return audio
@@ -41,6 +45,9 @@ const handler: NextApiHandler<Data> = async (req, res) => {
             : res.status(500).json({ error: "An error occurred while retrieving the audio" });
     }
 
+    if (!isValid(source))
+        return res.status(400).json({ error: "Invalid source language" });
+
     const textScrape = await googleScrape(source, target, query);
 
     if ("errorMsg" in textScrape)
diff --git a/pages/api/v1/languages/[[...slug]].ts b/pages/api/v1/languages/[[...slug]].ts
index ebc7b3d..b2fa0e4 100644
--- a/pages/api/v1/languages/[[...slug]].ts
+++ b/pages/api/v1/languages/[[...slug]].ts
@@ -1,10 +1,10 @@
 import { NextApiHandler } from "next";
 import NextCors from "nextjs-cors";
-import { retrieveFromType } from "@utils/language";
+import { retrieveFromType, LangCode } from "@utils/language";
 
 type Data = {
     languages: {
-        code: string,
+        code: LangCode,
         name: string
     }[]
 } | {
diff --git a/tests/pages/[[...slug]].test.tsx b/tests/pages/[[...slug]].test.tsx
index fafacd5..7c85bab 100644
--- a/tests/pages/[[...slug]].test.tsx
+++ b/tests/pages/[[...slug]].test.tsx
@@ -12,8 +12,8 @@ beforeEach(() => {
 });
 
 describe("getStaticProps", () => {
-    const source = faker.random.locale();
-    const target = faker.random.locale();
+    const source = "es";
+    const target = "ca";
     const query = faker.random.words();
 
     it("returns home on empty params", async () => {
diff --git a/tests/pages/api/graphql.test.ts b/tests/pages/api/graphql.test.ts
index f0c8859..7e418b4 100644
--- a/tests/pages/api/graphql.test.ts
+++ b/tests/pages/api/graphql.test.ts
@@ -50,7 +50,7 @@ it("returns translation triggering fetch", async () => {
 });
 
 it("returns audio triggering fetch", async () => {
-    const lang = faker.random.locale();
+    const lang = "es";
     const text = faker.random.words();
     resolveFetchWith({ status: 200 });
 
@@ -93,7 +93,7 @@ it("returns null and throws on translation error", async () => {
 });
 
 it("returns null and throws on audio error", async () => {
-    const lang = faker.random.locale();
+    const lang = "es";
     const text = faker.random.words();
     fetchMock.mockRejectOnce();
 
@@ -178,7 +178,7 @@ it("throws error on empty query in translation", async () => {
 });
 
 it("throws error on empty lang or query in audio", async () => {
-    const lang = faker.random.locale();
+    const lang = "es";
     const text = faker.random.words();
 
     const { errors: queryErrors } = await query({
diff --git a/tests/pages/api/v1/[[...slug]].test.ts b/tests/pages/api/v1/[[...slug]].test.ts
index edea1c2..465fb84 100644
--- a/tests/pages/api/v1/[[...slug]].test.ts
+++ b/tests/pages/api/v1/[[...slug]].test.ts
@@ -7,8 +7,8 @@ beforeEach(() => {
     fetchMock.resetMocks();
 });
 
-const source = faker.random.locale();
-const target = faker.random.locale();
+const source = "es";
+const target = "ca";
 const query = faker.random.words();
 const slug = [source, target, query];
 
diff --git a/tests/utils/language.test.ts b/tests/utils/language.test.ts
index 186c817..22bc5ab 100644
--- a/tests/utils/language.test.ts
+++ b/tests/utils/language.test.ts
@@ -13,7 +13,7 @@ describe("replaceBoth", () => {
         langType: LangType
     ) => (
         Object.entries(checkObj[langType]).forEach(([code, replacement]) => {
-            const res = replaceBoth(checkType, { source: "", target: "", [langType]: code })
+            const res = replaceBoth(checkType, { source: "auto", target: "en", [langType]: code })
             expect(res[langType]).toBe(replacement);
         })
     );
diff --git a/tests/utils/reducer.test.ts b/tests/utils/reducer.test.ts
index b09903b..4d73ec0 100644
--- a/tests/utils/reducer.test.ts
+++ b/tests/utils/reducer.test.ts
@@ -17,13 +17,13 @@ it("changes a field value", () => {
 it("changes all fields", () => {
     const query = faker.random.words();
     const state = {
-        source: faker.random.locale(),
-        target: faker.random.locale(),
+        source: "zh",
+        target: "zh_HANT",
         query,
         delayedQuery: query,
         translation: faker.random.words(),
         isLoading: faker.datatype.boolean()
-    };
+    } as const;
 
     const res = langReducer(initialState, {
         type: Actions.SET_ALL,
@@ -37,7 +37,7 @@ it("switches target on source change", () => {
         ...initialState,
         source: "es",
         target: "ca"
-    };
+    } as const;
 
     const res = langReducer(state, {
         type: Actions.SET_FIELD,
@@ -57,7 +57,7 @@ it("switches the languages & the translations", () => {
         target: "ca",
         query: faker.random.words(),
         translation: faker.random.words()
-    };
+    } as const;
 
     const res = langReducer(state, { type: Actions.SWITCH_LANGS });
     expect(res).toStrictEqual({
@@ -75,7 +75,7 @@ it("resets the source while switching if they're the same", () => {
         ...initialState,
         source: "eo",
         target: "eo"
-    };
+    } as const;
 
     const res = langReducer(state, { type: Actions.SWITCH_LANGS });
     expect(res.source).toStrictEqual(initialState.source);
diff --git a/tests/utils/translate.test.ts b/tests/utils/translate.test.ts
index a54401d..3dda897 100644
--- a/tests/utils/translate.test.ts
+++ b/tests/utils/translate.test.ts
@@ -2,8 +2,8 @@ import { htmlRes, resolveFetchWith } from "@tests/commonUtils";
 import faker from "faker";
 import { googleScrape, extractSlug, textToSpeechScrape } from "@utils/translate";
 
-const source = faker.random.locale();
-const target = faker.random.locale();
+const source = "es";
+const target = "ca";
 const query = faker.random.words();
 
 describe("googleScrape", () => {
diff --git a/utils/language.ts b/utils/language.ts
index 01327a7..52426f9 100644
--- a/utils/language.ts
+++ b/utils/language.ts
@@ -1,6 +1,8 @@
 import languagesJson from "./languages.json";
 const { languages, exceptions, mappings } = languagesJson;
 
+export type LangCode = keyof typeof languages;
+
 const checkTypes = {
     exception: exceptions,
     mapping: mappings
@@ -20,10 +22,10 @@ const isKeyOf = <T extends object>(obj: T) => (key: keyof any): key is keyof T =
 export function replaceBoth(
     checkType: CheckType,
     langs: {
-        [key in LangType]: string
+        [key in LangType]: LangCode
     }
 ): {
-    [key in LangType]: string
+    [key in LangType]: LangCode
 } {
     const [source, target] = langTypes.map(langType => {
         const object = checkTypes[checkType][langType];
@@ -33,8 +35,8 @@ export function replaceBoth(
     return { source, target };
 }
 
-export function retrieveFromType(type?: LangType): [string, string][] {
-    const langEntries = Object.entries(languages);
+export function retrieveFromType(type?: LangType) {
+    const langEntries = Object.entries(languages) as [LangCode, string][];
 
     if (!type)
         return langEntries;
@@ -43,6 +45,10 @@ export function retrieveFromType(type?: LangType): [string, string][] {
     ));
 }
 
-export function getName(code: string): string | null {
-    return isKeyOf(languages)(code) ? languages[code] : null;
+export function isValid(code: string | null | undefined): code is LangCode {
+    return !!code && isKeyOf(languages)(code);
+}
+
+export function getName(code: string): string | null {
+    return isValid(code) ? languages[code] : null;
 }
diff --git a/utils/reducer.ts b/utils/reducer.ts
index c859e36..a8e4647 100644
--- a/utils/reducer.ts
+++ b/utils/reducer.ts
@@ -1,16 +1,26 @@
-import { replaceBoth } from "./language";
+import { replaceBoth, isValid, LangCode } from "./language";
 
-export const initialState = {
-    source: "auto",
-    target: "en",
+const defaultSourceLang = process.env["NEXT_PUBLIC_DEFAULT_SOURCE_LANG"];
+const defaultTargetLang = process.env["NEXT_PUBLIC_DEFAULT_TARGET_LANG"];
+
+type State = {
+    source: LangCode,
+    target: LangCode,
+    query: string,
+    delayedQuery: string,
+    translation: string,
+    isLoading: boolean
+}
+
+export const initialState: State = {
+    source: isValid(defaultSourceLang) ? defaultSourceLang : "auto",
+    target: isValid(defaultTargetLang) ? defaultTargetLang : "en",
     query: "",
     delayedQuery: "",
     translation: "",
     isLoading: true
 }
 
-type State = typeof initialState;
-
 export enum Actions {
     SET_FIELD,
     SET_ALL,
@@ -32,7 +42,7 @@ type Action = {
     type: Actions.SWITCH_LANGS
 }
 
-export default function reducer(state: State, action: Action) {
+export default function reducer(state: State, action: Action): State {
     const { source, target } = replaceBoth("exception", {
         source: state.target,
         target: state.source
diff --git a/utils/translate.ts b/utils/translate.ts
index 4cc73ed..63acdf7 100644
--- a/utils/translate.ts
+++ b/utils/translate.ts
@@ -1,10 +1,10 @@
 import UserAgent from "user-agents";
 import cheerio from "cheerio";
-import { replaceBoth } from "./language";
+import { replaceBoth, LangCode } from "./language";
 
 export async function googleScrape(
-    source: string,
-    target: string,
+    source: LangCode,
+    target: LangCode,
     query: string
 ): Promise<{
     translationRes: string
@@ -57,8 +57,8 @@ export function extractSlug(slug: string[]): {
     }
 }
 
-export async function textToSpeechScrape(lang: string, text: string) {
-    const { target: parsedLang } = replaceBoth("mapping", { source: "", target: lang });
+export async function textToSpeechScrape(lang: LangCode, text: string) {
+    const { target: parsedLang } = replaceBoth("mapping", { source: "auto", target: lang });
 
     const lastSpace = text.lastIndexOf(" ", 200);
     const slicedText = text.slice(0, text.length > 200 && lastSpace !== -1 ? lastSpace : 200);