Quick gut feel answer - IMO it should be localized in local, monolingual project invocations of media viewer (dewiki, frwiki, etc.) and canonical on multilingual wikis (Meta, Commons). If you're in dewiki and frwiki and copy wikitext, you'll most likely want to use it on that wiki, and the common expectation is to use the localized namespace prefix. If you're doing cross-wiki work, you can either go to Commons or adapt the namespace prefix. Optimize for the common case.

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Erik Möller
VP of Engineering and Product Development, Wikimedia Foundation