Automate translation integration

[kueda]

• Experiment with https://github.com/crowdin/github-action to upload source files when they change
• We also want to download translations every month (e.g. on the 7th), but never upload translations

Since we don't have any tests, I think it would be ok to add them directly to the main branch

[alexshepard]

We do have a few tests, and I also manually test translations before I download and integrate them - I'll see if I can convert that python integration test into an ObjC or Swift test.