chore(i18n): configure Elgg for Transifex support

1. Add all language files to .tx/config

2. Remove comments from the top of language files.

   Transifex does not strip comments from the source files
   so they end up looking tacky when they say "English file"
   and they're actually in a different language.

3. Converted old `add_translation("en", array(...))` calls
   into the new `return array(...)` format.

   This is for a similar reason as in point 2.

Fixes #6709
chore(i18n): update translations