GZDoom is in the transition of its localized string handling right now.
The goal is to have the language resources in a place where it can be edited by external contributors that do not use the GZDoom Github project directly - they are hosted on Google Docs so that anyone with interest to contribute can request access to edit.
In the future the language.enu lump will no longer exist, instead, all texts will be read from a spreadsheet file within gzdoom.pk3 which is periodically updated from the public versions mentioned above.
This has a few implications on how text content needs to be handled - editing the language files within the GZDoom project is no longer possible because they no longer represent the master version of the text resources. For pull requests this means that they have to do a minor bit of added work to get the newly added strings to where they belong: Instead of editing language.enu, the new texts will have to be added to the PR as a comment, outside the actual changeset so that if the PR gets merged they can be added to the spreadsheet easily.
Please note that since the text data in the Github repo is not the master resource, any alteration of it by a PR needs to be rejected without exception.