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