PW 3.0.165, installed languages English, German.
I've solved it, but I don't know what could possibly have caused this: "Out of the blue" (well...) my PW backend switched from German to English.
The language option in the user profile menu had vanished. Field "language" had disappeared from template "user" and from the template's list of selectable fields. After some digging I found out that the entry which links the field "language" (fields.id=98) to fieldgroup "user" (fieldgroups.id=3) in fieldgroup_fields had vanished, too. Entering a new record (fieldgroup_fields.fieldgroups_id=3, .fields_id=98, .sort=[next index available]) solved the problem. (Fortunately, fieldgroup_fields has no id of its own, on which might depend - whatever.)
Now, who or what erased this record?! And could I have solved this whithout tampering with the tables?