I think this doesn't actually block 30217, since the current mechanism would allow such a fallback loop to work (actually, it'd be treated as a chain and would be cutoff after 5 iterations, in case none of the localizations exist).

Except that the cutoff is an exception which halts code execution :/

Created attachment 8890
Proposed patch

Here is patch that makes the fallbacks a flat list instead of recursive chains. This way we avoid the problems with cycles, because we don't have them anymore.

Not included in the patch: updating $fallback in message files nor updating callers of getFallbackFor (which I plan to deprecate, perhaps with some heuristics to avoid causing loops for callers)

Attached:

r94907.