How to find the number of papers per language written by a researcher?
Clarivate's Web of Science citation database can provide this information. Assuming you have access to it, e.g. through your institution, you can


Search for a researcher's name
If necessary, use additional criteria to try to restrict the results to a single person
Look under "More options" for languages.


While this option is mainly intended for further restricting the search results, the tool also tells you the number of articles in language X associated with researcher name Y. Of course, their database coverage has some restrictions, so it may or may not work for your purposes. In certain fields, other databases could be more useful.