On enwiki the Babel categories are in the format of "Category:User xx", where xx is always in lower case. However, if a user enters codes with a different capitalisation in their #babel invocation, the page is categorised with that different capitalisation.
For example, on enwiki, the code {{#babel: En}} will add the page to the category "Category:User En", when it should be "Category:User en".
This also has the consequence that [[User:Babel AutoCreate]] creates duplicate categories for each different code capitalisation that someone has used. I see it has created both [[:Category:User Zh]] and [[:Category:User zh]], for instance. (I have blocked the Babel AutoCreate account on enwiki until we can find a way to fix this.)
I assume that lower case is the preferred code format for other wikis, and that seemed to be the case when I spot-checked a few of them. However, if any wikis use a different system, code capitalisation might need to be made configurable, rather than lower case being hard-coded.
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