bug 17592 lends itself towads the idea that the fallback language for both nds-NL and nds-DE localizations could
or should become nds rather than current nl and de.

Discussion also revealed that indeed:

nds-NL → nds → nl → (en)

and

nds-DE → nds → de → (en)

was the best possible choice. This is backed by the recent two
decades of assement in scientific literature of the language
groups involved, which is also reflected at least in the
corresponding articles in the German Wikipedia (did not
check any other ones)

(By the way, we have, or shall likely soon encounter, similar
complexities with other language groups)

Such fallback paths are currently impossible, since we have only
one single fallback language per language. Yet, the concept can
pretty easily be expanded by allowing fallback languages to be
given as an array of language codes, too, which we should do.
Array members are to be checked in the given order, before any
of their own fallback languages are, and this may recurse.
No language is to be checked a second time, obviously, and we
can still keep the limit of at most 5 languages (imho), thus not
sacrificing any processing time.

Version: 1.15.x
Severity: enhancement