It is sometimes useful to map one language code to several actual languages. This is particularly relevant with macro languages and with regional variants.
For example, the Mari language of Russia is listed as "chm" (macro) in the CLDR Territoty-Language table, but Wikipedia uses the particular codes mhr and mrj. Another issue is Belarusian—the CLDR table uses only "be" for Belarus, but Wikipedia uses "be" and "be-tarask".
There are almost certainly more cases—I haven't gone over the whole languages list. There can be issues with Azerbaijani, Punjabi, Chinese, Norwegian and more languages.
ULS currently doesn't support this, but we should add it, at least for some cases.
(In ContentTranslation, the split of "be" to "be" and "be-tarask" is hard-coded, and it also does weird tricks with Norwegian. This is very far from being bulletproof. We should fix it more thoroughly.)