Update po files and add German translation

[daPhipz]

I added a German translation and updated all po files first by running make update-po-files.

Feel free to ping me whenever there are new strings to translate!

[ghost]

Please refrain from updating the other languages unless there's a good reason, as it may block other translators from updating their translations.

[jorchube]

Hello @daphipz

Thank you so much for your work!

You may want to consider adding yourself to src/translators.py :)

[jorchube]

Please refrain from updating the other languages unless there's a good reason, as it may block other translators from updating their translations.

I see that this is my fault for adding that update-po-files make target (I'll remove it). @sabriunal I will take care when merging this PR to not affect other translators work. :)