It looks like the plugin is calling the API more often that expected, only to detect in which language the post is written.
I discovered that because my free 500KB quota on DeepL has been exhausted in a couple of days only on an instance I was barely using.
There is another option to detect post languages: GitHub - peterc/whatlanguage: A language detection library for Ruby that uses bloom filters for speed.
No API, only local
I’ve also sent a mail to DeepL to see if they have special offers for free / open projects… it cost nothing to ask 