Language files and own translations

Hi,
Booster comes with a German language file. This is the “normal” language. In Germany there is a second “formal/official” language that I use with WP. So to make that translation work I had to copy and rename the original booster-german-language-file using loco translate.
But now the issue: If you want to make your own translations or modify them update-save loco translate gives the possibility to copy the base file to a new folder. Doing that with other plugins wordpress takes the new file for translation. But Booster doesn’t! Translations in the copied custom file don’t work. Booster always is connected to the author-language-file.
Any ideas?
Thanks in advance
Koepi