The best way would be to join the translation team on transifex. There you can join the team and translate strings for different apps. That way, the improved translation goes into official code and you don’t have to go through everything manually (you have to redo this after every update, for larger updates the strings can change a bit, so it’s not enough to just copy your modified version all the time).
In theory, it should be enough to modify the files by hand on your system. Not sure about docker, if you can easily modify code.
Here is a wiki about the translation stuff: Translation knowledge (valid for the entire Nextcloud project) - Wiki