merge translations into a single file per language

[doniks]

for context see: #628

[doniks]

added a PR to try to do this: #631

as far as I can see, jenkins CI only builds the english version, so that's no good to test whether the translation setup works ok

I thiiink, I can ask rtd to build this branch. I've added en and de for now to to test:

• EN: https://docs.ubports.com/en/merge_po
• DE: https://docs.ubports.com/de/merge_po

So, that doesn't work yet, DE version is all EN. which doesn't really surprise me since I probably need to figure out how sphinx will pick up the translations from the new merged file ....

[doniks]

ooookay ... I guess I'm kinda there ...

[doniks]

alright. seems to look ok. @Danfro helped me with a weblate smoke test (with locked weblate component), where a reasonable number of languages and translated strings showed up