fix(translation): resolve DeepL translation failure with auto-detection (#3412)

* fix(translation): correctly handle DeepL source_lang 'AUTO' by omitting it

* refactor: backward compatibility

---------

Co-authored-by: Huang Xin <chrox.huang@gmail.com>

apps/readest-app/src/pages/api/deepl/translate.ts

@@ -183,12 +183,21 @@ async function callDeepLAPI(
   // TODO: this should be processed in the client, but for now, we need to do it here
   // please remove this when most clients are updated
   const input = text.replaceAll('\n', '').trim();
-  const requestBody = {
+
+  const requestBody: {
+    text: string | string[];
+    target_lang: string;
+    source_lang?: string;
+  } = {
     text: isV2Api ? [input] : input,
     source_lang: isV2Api ? sourceLang : (LANG_V2_V1_MAP[sourceLang] ?? sourceLang),
     target_lang: isV2Api ? targetLang : (LANG_V2_V1_MAP[targetLang] ?? targetLang),
   };
 
+  if (isV2Api && requestBody.source_lang?.toUpperCase() === 'AUTO') {
+    delete requestBody.source_lang;
+  }
+
   const response = await fetch(apiUrl, {
     method: 'POST',
     headers: {

apps/readest-app/src/services/translators/providers/deepl.ts

@@ -36,9 +36,10 @@ export const deeplProvider: TranslationProvider = {
       throw new Error('Authentication token is required for DeepL translation');
     }
 
+    const normalizedSourceLang = normalizeToShortLang(sourceLang).toUpperCase();
     const body = JSON.stringify({
       text: text,
-      source_lang: normalizeToShortLang(sourceLang).toUpperCase(),
+      ...(normalizedSourceLang !== 'AUTO' ? { source_lang: normalizedSourceLang } : {}),
       target_lang: normalizeToShortLang(targetLang).toUpperCase(),
       use_cache: useCache,
     });