My git project (a Wordpress-Plugin) contains some 3rd party submodules with extra files (language .po files, /tests folder, coverage.cover) that are not needed in release versions.
Is there a command/workflow that strips all of these files automatically for releases?
The release management process differs from the source control management (here Git and its submodule)
If your deployment process is to simply checkout the content of a Git repo (and its submodule, with a git submodule update --init --recursive), then no, there is nothing native to Git that would strip out the non-needed part of those subrepos.
You would need to call an additional script (made for fine-tuning the deployment), which would take care of that step.