That is really strange, as Digi Netatmo has translatable strings feature.
Is your Joomla German language tag de-DE?
Do you see all the module strings in English or only the weather measurements? I mean, the backend of the module is in German or in English? And you see in English all the frontend data?
I then renamed the it-IT language file to en-GB and copied it to the en-GB language folder.
Since then all Netatmo modules and texts of the edition were displayed in Italian.
Only with the German language file does not work with me.