Improve download translation script

[rffontenelle]

The reasons are explained in the commits, but here is a summary:

• Fix issues in my previous "variables rename" commit
• Fix verification of binary availability, and include check for msguniq as gettext package could not be available
• Make LANGUAGES_UPDATED variable exclusively for updated translation files, changing the final info output for that fact
• Add language codes from Translation Project that are not listed in the script
• Add prefixes ::error::some error text here and ::warning::some warning here to include annotation when running in GitHub Actions (which is a WIP right now)

[ensonic]

with this you can actually write:
diff --version 2>/dev/null >/dev/null | err_msg "==== You must have the 'diff' program installed for this script ===="

(also below)

[ensonic]

If you want, we can also omit the "===="

[rffontenelle]

Done, reduced some lines.

[ensonic]

thanks a lot!