>After a while I concluded it must be a defensive mechanism designed to detect and repel foreigners. As evidence I adduce the fact that some words change gender

>After a while I concluded it must be a defensive mechanism designed to detect and repel foreigners. As evidence I adduce the fact that some words change genders between regions. Well-known example: die Butter and der Butter depending on where you are from. It's not uncommon for a noun to change their gender at some point in history. As far as I know, it's often analogical changes. For example, feminine noun X describes an item of a larger class Y, and it just happens that most items of class Y are masculine. So the noun X, being feminine, is an exception, which is "fixed", and it becomes masculine. An example from Russian language: most words denoting shoes and related items were masculine: boot, shoe, sock etc. Then there was the word for "slippers", feminine. I and my family still use the word in the feminine form, because the change is very recent. Today most speakers (especially younger ones) use the masculine form. It's expectable that a change can happen in some dialects, but not in others.