translate: ignore a peer answer that names the target as its source language

packages/translate/src/translate.js

@@ -123,7 +123,7 @@ export async function translatePost(input) {
     },
     read: (text) => {
       const parsed = readTagged(text, ['title', 'summary', 'source_language']);
-      return parsed && finishPost(parsed);
+      return parsed && finishPost(fixSourceLanguage(parsed, input));
     },
   });
 }
@@ -310,7 +310,7 @@ export async function translateArticle(input) {
     read: (text) => {
       const parsed = readTagged(text, ['title', 'summary', 'source_language', 'body']);
       if (!parsed) return null;
-      const result = finishArticle({ ...parsed, content_html: parsed.body });
+      const result = finishArticle({ ...fixSourceLanguage(parsed, input), content_html: parsed.body });
       return result && { ...result, truncated: peerCut };
     },
   });
@@ -423,6 +423,23 @@ export function readTagged(text, fields) {
   return out;
 }
 
+/**
+ * A small model often reports the language it wrote in (the target) as the
+ * source. When it names the target, trust the feed's declared language
+ * instead, or record none: "translated from English into English" is wrong
+ * either way.
+ *
+ * @param {Record<string, string>} parsed
+ * @param {{ targetLang: string, sourceLang?: string|null }} input
+ */
+export function fixSourceLanguage(parsed, input) {
+  const base = (/** @type {unknown} */ v) => String(v ?? '').trim().toLowerCase().split(/[-_]/)[0];
+  if (parsed.source_language && base(parsed.source_language) === base(input.targetLang)) {
+    return { ...parsed, source_language: input.sourceLang ? String(input.sourceLang) : '' };
+  }
+  return parsed;
+}
+
 /**
  * Cuts HTML at a paragraph-ish boundary at or before `max` characters, so the
  * model is not handed half a tag.

packages/translate/test/translate.test.js

@@ -201,3 +201,10 @@ test('readTagged reads the short fields and a body full of markup', async () =>
   assert.equal(readTagged('<title>x</title>', ['title', 'summary']), null);
   assert.equal(readTagged('<title>x</title><body><p>cut off', ['title', 'body']), null);
 });
+
+test('a peer answer that names the target as its source falls back to the declared language', async () => {
+  const { fixSourceLanguage } = await import('../src/translate.js');
+  assert.equal(fixSourceLanguage({ source_language: 'en' }, { targetLang: 'en', sourceLang: 'de' }).source_language, 'de');
+  assert.equal(fixSourceLanguage({ source_language: 'EN-us' }, { targetLang: 'en' }).source_language, '');
+  assert.equal(fixSourceLanguage({ source_language: 'fr' }, { targetLang: 'en', sourceLang: 'de' }).source_language, 'fr');
+});