diff --git a/src/login/i18n/noJsx/GenericI18n_noJsx.ts b/src/login/i18n/noJsx/GenericI18n_noJsx.ts
index a5abce2e..1a0afb30 100644
--- a/src/login/i18n/noJsx/GenericI18n_noJsx.ts
+++ b/src/login/i18n/noJsx/GenericI18n_noJsx.ts
@@ -16,7 +16,7 @@ export type GenericI18n_noJsx<MessageKey extends string, LanguageTag extends str
      * Used to render a select that enable user to switch language.
      * ex: https://user-images.githubusercontent.com/6702424/186044799-38801eec-4e89-483b-81dd-8e9233e8c0eb.png
      * */
-    labelBySupportedLanguageTag: Record<string, string>;
+    labelBySupportedLanguageTag: Record<string /*LanguageTag*/, string>;
     /**
      *
      * Examples assuming currentLanguageTag === "en"
diff --git a/src/login/i18n/noJsx/getI18n.tsx b/src/login/i18n/noJsx/getI18n.tsx
index 3ce51695..679721c2 100644
--- a/src/login/i18n/noJsx/getI18n.tsx
+++ b/src/login/i18n/noJsx/getI18n.tsx
@@ -7,7 +7,11 @@ import { FALLBACK_LANGUAGE_TAG } from "keycloakify/bin/shared/constants";
 import { id } from "tsafe/id";
 import { is } from "tsafe/is";
 import { Reflect } from "tsafe/Reflect";
-import type { LanguageTag as LanguageTag_defaultSet, MessageKey as MessageKey_defaultSet } from "../messages_defaultSet/types";
+import {
+    type LanguageTag as LanguageTag_defaultSet,
+    type MessageKey as MessageKey_defaultSet,
+    languageTags as languageTags_defaultSet
+} from "../messages_defaultSet/types";
 import type { GenericI18n_noJsx } from "./GenericI18n_noJsx";
 
 export type KcContextLike = {
@@ -49,6 +53,17 @@ export function createGetI18n<
 }): ReturnTypeOfCreateGetI18n<MessageKey_themeDefined, LanguageTag_notInDefaultSet> {
     const { extraLanguageTranslations, messagesByLanguageTag_themeDefined } = params;
 
+    Object.keys(extraLanguageTranslations).forEach(languageTag_notInDefaultSet => {
+        if (id<readonly string[]>(languageTags_defaultSet).includes(languageTag_notInDefaultSet)) {
+            throw new Error(
+                [
+                    `Language "${languageTag_notInDefaultSet}" is already in the default set, you don't need to provide your own base translations for it`,
+                    `If you want to override some translations for this language, you can use the "withCustomTranslations" method`
+                ].join(" ")
+            );
+        }
+    });
+
     type LanguageTag = LanguageTag_defaultSet | LanguageTag_notInDefaultSet;
 
     type MessageKey = MessageKey_defaultSet | MessageKey_themeDefined;