[meta] Improve translation robustness

[eu9ene]

This is a meta issue for tracking issues to improve the robustness of the translation models:

### Tasks
- [ ] https://github.com/mozilla/firefox-translations-training/issues/514
- [ ] #215
- [ ] #177
- [ ] https://github.com/mozilla/firefox-translations-training/issues/736
- [ ] https://github.com/mozilla/firefox-translations-training/issues/903
- [ ] https://github.com/mozilla/firefox-translations-training/issues/737
- [x] #161
- [ ] #73
- [ ] #129
- [ ] https://github.com/mozilla/firefox-translations-training/issues/262
- [ ] https://github.com/mozilla/firefox-translations-training/issues/261
- [ ] #176
- [ ] #173
- [ ] #171
- [ ] #69
- [ ] https://github.com/mozilla/firefox-translations-training/issues/411
- [ ] https://github.com/mozilla/firefox-translations-training/issues/577
- [ ] https://github.com/mozilla/firefox-translations-training/issues/255
- [ ] https://github.com/mozilla/firefox-translations-training/issues/167
- [ ] https://github.com/mozilla/firefox-translations-training/issues/600
- [ ] https://github.com/mozilla/firefox-translations-training/issues/757
- [ ] https://github.com/mozilla/firefox-translations-training/issues/800
- [ ] https://github.com/mozilla/firefox-translations-training/issues/811