Hello.
It seems that some code was inserted in an uneditable translation page [https://commons.wikimedia.org/w/index.php?title=Commons:Copyright_rules_by_territory/Serbia/en&diff=prev&oldid=889805020]
when I performed a deletion in Wikimedia Commons
Is this a known bug?
Not exactly a known bug, but you're doing something that isn't supported properly by Translate.
What you should have done, after deleting the content from https://commons.wikimedia.org/wiki/Commons:Copyright_rules_by_territory/Serbia, was marked the page for translation, which would have automatically deleted the content from the /en page (and all other translation). That requires "translation administrator" rights, but admins who know what they're doing can grant themselves such rights. (Translation administration is kind of a powertool and misuse of it can create messes that are much harder to cleanup than they are to make)
Editing or deleting individual translation units corresponding to the source text is not an operation supported by Translate, and should be prevented outright (T366425).
Thanks a lot for the reply and instructions! I have now implemented this on Commons. I agree that the operation should be prevented outright and it should be clearly documented how to go about this.
T366425 means that translate will now tell you you're screwing up if you try to do this rather than behaving in mysterious and confusing ways.