Translated language files are not loaded
If extension's or core language file contains section for another language (either partial or full) and external translation file exists in language pack (possibly more complete), than that external file will never be loaded. It is enough to have even empty section for non-default language to prevent external translation files to be loaded.
According to Kasper, all translations must be moved to external files now to minimize amount of loaded strings and load only necessary translations from TER.
The attached patch gives priority to external file if it exists. This is a change in concept and should be approved by core team.
(issue imported from #M6903)