Steps to reproduce:

1. Log in to any Wikisource. Change your preferences such that your language is not that of the host wiki.
2. Edit any Page for proofreading.
3. Change the page status. Observe that the generated edit summary is in the language that is in my preferences, not the host wiki.
4. Save the page. The edit summary for that revision is in the language that is in my preferences, not the host wiki.

It is more sensible to have the edit summary be in the language of the host wiki so that native speakers can understand it.

Example:
https://en.wikisource.org/w/index.php?title=Page%3AFurcountryorseve00vernrich.djvu%2F130&type=revision&diff=9836674&oldid=7054215