Further multilanguage support improvements

[jtherczeg]

The great latest localization enhancements for PW 2.5 raise some problems due to the multi-language site profile.

I observed that when the site translation files are included in the core translation files language pack, they are unnecessarily uploaded to the Core Translation Files and I can delete them, but cannot move them to their appropriate location.

As the translation files are distributed in one package, many of them downloaded directly from GitHub, it would be a smarter solution if the language pack uploader dispatched the site language files to the Site Translation Files section.

It turned out in Ryan's answer on my GitHub issue report, that we cannot use these site translation files for monolingual sites, so it would be recommended if the language pack uploader detected the multilingual status and when it failed (due to monolingual site), it displayed a warning and did not upload the site language files.