Hi Ralf!
My apologies for the late reply (I was on holidays! :-S).
The .json files included in the H5P content-type libraries are updated by the H5P team every time they release a new content-type library version. So, for instance, if you've been translating to German the "Interactive video" content-type sending them a pull-request to the main repository (https://github.com/h5p/h5p-interactive-video/blob/master/language/de.json), it will be included once the H5P team approve your changes and a new interactive video library version will be released. For now, this is the recommended way to follow in order to install/update H5P content-type translations.
Does this answer your question? Or are you talking about a different translation (as there are several points to translate related to H5P, maybe I've misunderstood you)?
Kind regards!