Whatever the solution behind is, if you have the resources, move to something self-hosted. Open core or not, if that topic matters to you, you might need something you can own and control. BTW, have a look on Forgejo, Codeberg and Gitea: these are the solutions I see when people look for something FLOSS, not open core, and maybe self-hostable.

It depends of the project in fact. You should reach the community and maintainers by joining them in their Discord / Slack / Matrix / whatever. They may be able to help you.

You can create first an issue, asking for improvements and create a discussion airy the maintainers so as to know which languages are not managed yet and if they are interested in new support. Explains also why you can bring good translations (e.g. native speaker, teacher, etc). It sill help to bring confidence.

Then create a pull / merge request with the updated files. For example, strings.xml ob Android, .strings in iOS, etc. But beware, localisation is not only a matter of translations. You may have also to support new languages and formats for figures, currencies, or dates for example.

Do not use translations services. Project maintainers are able to use them, and in plenty of cases the translations are not good at all or loose details.

I would have said in fact Matrix or XMPP-based solutions but it seems you already have spotted them. Maybe Mattermost?

It seems yet Bluesky has an Android app according to their GitHub repository (https://github.com/bluesky-social/social-app). However indeed nothing mentioned about F-Droid ; maybe some alternative Mastodon clients can also deal with Bluesky?

GitLab because for CI/CD is it far, far much user friendly and comfortable to use with GitLab CI compared to GitHub Actions and flows.

In addition I can integrate templates for CI/CD pipelines already defined with the To Be Continuous project (which is open source).

https://to-be-continuous.gitlab.io/doc/