Steps to replicate the issue (include links if applicable):

As seen in T409892: Broken namespaces in the Buginese Wikipedia:

• Have a language that has a non-English $fallback language defined in its MessagesXxx.php file, but doesn't have the namespace names defined
• Create a wiki in that language, and make various edits. Notably, add some images and categories, and post some talk page messages.
• Then add namespace definitions for that language.

What happens?:

• Before the language has namespace names defined, it will use the namespace names from the fallback language (in T409892, that was Indonesian)
• When namespace names are added for the language in question, the old links that use the namespace names from the fallback language stop working.

What should have happened instead?:

• The namespace names from all fallback languages should automatically be included as namespace aliases, so that links will continue working.

Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):