Core

Imagine an old extension back from the 4.5-days, whose language file comes in two flavors:

en (default)
dk (old format, <= 4.5)

To properly support the new translations in 4.6, the site config is as follows:

language = da
language_alt = dk

However, even though the "dk" key is present in the language file for the extension in question, it seems to be completely ignored. And since the language key for Danish was changed from "dk" to "da" between 4.5 and 4.6, this functionality might need an overhaul so it'll work as expected. :-)