fix(i18n): use translate key as fallback

- If the translate key is nonsense (not seen in any of the languages)
then the translate key as-is should be returned as value, this helps
during development. Currently it displays the first entry of the locale
store which is "Home".
- Regression from forgejo/forgejo#6203.
- Added unit test.

modules/translation/i18n/i18n_test.go

@@ -158,6 +158,7 @@ commits = fallback value for commits
 
 	found := lang1.HasKey("no-such")
 	assert.False(t, found)
+	assert.EqualValues(t, "no-such", lang1.TrString("no-such"))
 	require.NoError(t, ls.Close())
 }