diff --git a/.storybook/index.mdx b/.storybook/index.mdx
index 38b141c..934f046 100644
--- a/.storybook/index.mdx
+++ b/.storybook/index.mdx
@@ -48,6 +48,14 @@ declare module '@douglasneuroinformatics/libui/i18n' {
 init({ translations: { common } });
 ```
 
+To require that every language be provided, rather than falling back to the default language at runtime, add:
+
+```ts
+export interface Options {
+  requireCompleteTranslations: true;
+}
+```
+
 **main.tsx**
 
 ```js
diff --git a/TRANSLATIONS.md b/TRANSLATIONS.md
index d7ba341..79bc96d 100644
--- a/TRANSLATIONS.md
+++ b/TRANSLATIONS.md
@@ -16,7 +16,7 @@ The `Language` type includes `en`, `es`, and `fr` out of the box. Every leaf nod
 }
 ```
 
-All language keys are optional (`{ [L in Language]?: string }`). When the active language has no translation, the translator falls back to `defaultLanguage` (defaults to `en`).
+By default, all language keys are optional (`{ [L in Language]?: string }`). When the active language has no translation, the translator falls back to `defaultLanguage` (defaults to `en`). Consumers who want every language to be mandatory can opt in — see [Requiring complete translations](#requiring-complete-translations).
 
 ## Architecture
 
@@ -99,6 +99,45 @@ The `LanguageToggle` component renders a dropdown from the `options` prop — on
 <LanguageToggle options={{ en: 'English', fr: 'Français' }} />
 ```
 
+## Requiring complete translations
+
+By default a translation may omit languages and fall back at runtime. Set `requireCompleteTranslations` on `UserConfig.Options` to make every language in `LanguageOptions` mandatory:
+
+```ts
+declare module '@douglasneuroinformatics/libui/i18n' {
+  export namespace UserConfig {
+    export interface Options {
+      requireCompleteTranslations: true;
+    }
+  }
+}
+```
+
+With the flag set, both registered JSON namespaces and inline objects are checked:
+
+```ts
+// error: Property 'fr' is missing in type '{ en: string; es: string; }'
+t({ en: 'Save', es: 'Guardar' });
+```
+
+```ts
+// common.json is missing "fr" for one or more keys
+declare module '@douglasneuroinformatics/libui/i18n' {
+  export namespace UserConfig {
+    export interface Translations {
+      // error: Property 'common' ... is not assignable to 'string' index type
+      common: typeof common;
+    }
+  }
+}
+```
+
+The JSON error is reported on the offending property in your own `declare module` block, so it is unaffected by `skipLibCheck`. Note that:
+
+- Leaf **detection** stays permissive, so adding a language that `libui.json` does not yet translate never corrupts `TranslationKey`.
+- libui's own `libui` namespace is exempt from the check — it is typed directly from `libui.json` rather than through the index signature.
+- Per-language format arguments (`TranslateFormatArgs`) remain optional, since they are a formatting convenience rather than translated copy.
+
 ## Adding a new language
 
 1. Add the language code to `LanguageOptions` in `src/i18n/types.ts`.
diff --git a/src/i18n/translator.ts b/src/i18n/translator.ts
index a719deb..61fe616 100644
--- a/src/i18n/translator.ts
+++ b/src/i18n/translator.ts
@@ -10,6 +10,7 @@ import type {
   TranslateOptions,
   TranslationKey,
   Translations,
+  TranslationValue,
   TranslatorType
 } from './types.ts';
 
@@ -112,7 +113,7 @@ export class Translator implements TranslatorType<TranslationKey> {
   }
 
   @InitializedOnly
-  t(target: TranslationKey | { [L in Language]?: string }, { args }: TranslateOptions = {}): string {
+  t(target: TranslationKey | TranslationValue, { args }: TranslateOptions = {}): string {
     let obj: { [key: string]: string };
     if (typeof target === 'string') {
       obj = get(this.#config.translations, target) ?? {};
diff --git a/src/i18n/types.ts b/src/i18n/types.ts
index d2848ff..1d1c39a 100644
--- a/src/i18n/types.ts
+++ b/src/i18n/types.ts
@@ -7,7 +7,7 @@ import type { Merge, OmitIndexSignature, Primitive, Simplify } from 'type-fest';
 import type libuiTranslations from './translations/libui.json';
 
 interface TranslationsLike {
-  [key: string]: TranslationsLike | { [L in Language]?: string };
+  [key: string]: TranslationsLike | TranslationValue;
 }
 
 interface DefaultLanguageOptions {
@@ -16,10 +16,32 @@ interface DefaultLanguageOptions {
   fr: true;
 }
 
+/**
+ * The shape used to *identify* a translation leaf, as opposed to a group of nested translations.
+ * This is always partial, so that key extraction is unaffected by {@link RequireCompleteTranslations}.
+ */
+type TranslationValueLike = { [L in Language]?: string };
+
 export declare namespace UserConfig {
   interface LanguageOptions {
     [key: string]: boolean;
   }
+  /**
+   * Opt-in flags, set by consumers through declaration merging. This interface is intentionally
+   * empty here: declaring a member with a default (e.g. `requireCompleteTranslations?: boolean`)
+   * would make a consumer's `requireCompleteTranslations: true` an illegal redeclaration, since
+   * merged interface members must have identical types.
+   *
+   * @example
+   * declare module '@douglasneuroinformatics/libui/i18n' {
+   *   export namespace UserConfig {
+   *     export interface Options {
+   *       requireCompleteTranslations: true;
+   *     }
+   *   }
+   * }
+   */
+  interface Options {}
   interface Translations extends TranslationsLike {}
 }
 
@@ -27,6 +49,21 @@ export type LanguageOptions = OmitIndexSignature<Merge<DefaultLanguageOptions, U
 
 export type Language = keyof { [L in keyof LanguageOptions as LanguageOptions[L] extends true ? L : never]: any };
 
+/**
+ * Whether every language in {@link LanguageOptions} must be provided for each translation, as
+ * opposed to falling back to the default language at runtime. Set via `UserConfig.Options`.
+ */
+export type RequireCompleteTranslations = UserConfig.Options extends { requireCompleteTranslations: true }
+  ? true
+  : false;
+
+/**
+ * The shape a translation leaf must *satisfy*, whether it is defined inline or in a JSON file.
+ */
+export type TranslationValue = RequireCompleteTranslations extends true
+  ? { [L in Language]: string }
+  : TranslationValueLike;
+
 export type Translations = Simplify<
   OmitIndexSignature<UserConfig.Translations> & {
     libui: typeof libuiTranslations;
@@ -35,7 +72,7 @@ export type Translations = Simplify<
 
 export type ExtractTranslationKey<T extends { [key: string]: any }, Key = keyof T> = Key extends string
   ? T[Key] extends { [key: string]: any }
-    ? T[Key] extends { [K in Language]?: string }
+    ? T[Key] extends TranslationValueLike
       ? Key
       : `${Key}.${ExtractTranslationKey<T[Key]>}`
     : `${Key}`
@@ -60,7 +97,7 @@ export type TranslateOptions = {
 
 export interface TranslateFunction<TKey extends string> {
   (key: TKey, options?: TranslateOptions): string;
-  (translations: { [L in Language]?: string }, options?: TranslateOptions): string;
+  (translations: TranslationValue, options?: TranslateOptions): string;
 }
 
 export type TranslatorType<TKey extends string> = {