Adding new locales to Gettext projects directly from Pontoon is no longer possible

[mathjazz]

For Gettext projects, .po files must be included in the repository for each target locale for which they are to be translated (these files may be initially empty). For all other supported formats, Pontoon will automatically add files for each locale when it is translated.

The reason for this discrepancy is that Pontoon allows for different files to be enabled for different locales in Gettext projects, even if the TOML configuration files aren't used. However, this also means non-technical PMs cannot add new locales to Gettext projects directly from Pontoon.

See the discussion in https://github.com/mozilla/pontoon/issues/3573.

We should find the way forward. One option is to make sync less opinionated and require the use of TOML files for more complex file and folder structures like the one where different locales localize different sets of files.