Where to locate languages names in Wikipedia sidebar
Except for the autonyms (language names in these languages themselves), which are resolved in a static PHP class:
All translated language names come from CLDR data. For Western Frisian see (last published version is v37):
If translated names are missing, fallbacks are used, and so they will use the normal BCP47 rules for Western Frisian and will fallback to Dutch, otherwise to English.
To solve it, go to CLDR and start translating and vetting missing names (it is open today). After a new CLDR version will be published, and new names were vetted successfully, they will be loaded in Wikimedia with the CLDR data update...
Thanks, Verdy p. That is very helpful. This stuff is hidden pretty good. I don't think I would ever have found it on my own.
It's not hidden, but Wikimedia decided to not maintain these translations by itself. There are over 7000 languages in ISO 639 (not counting variants), and translating all pairs would require over 49 millions translations (this is a huge work: best to work with international contributors and not just inside Wikimedia or this small wiki), so fallbacks are used, and BCP 47 rules are used.
Note that CLDR still does not support localisation to all these languages, but it supports a lot (and at least those supported in this wiki and in Wikimedia). Other languages may be requested to the CLDR TC at Unicode. But you need convincing arguments (e.g. there exists a demonstrated need for contents, such as a wiki on Wikimedia or Omegawiki, or other softwares or text catalogs).
Some languages can also be requested here if they have a test wiki in Wikimedia Incubator (for that you need to contact the Wikimedia admins on Wikimedia Incubator, and in the Wikimedia Languages Commitee, but opening a new Incubator is easy; this does not mean that it can be done without taking care of the technical infrastructure of Wikimedia Incubaor; the Incubator has lot of test wikis that are not maintained and will never bring the creation of a new wiki edition for Wikipedia or Wiktionary).
But creating a test wiki in Incubator or Omegawiki can be done even before support for the language is added here. Once this test wiki starts being helpful, you can come here to request translting the UI and then contact Wikimedia Incubator admins or Omegawiki admins so that they import the translations made here, and separately you can request to CLDR TC to start translations too (but CLDR is very slow for its versioning, about one edition each year, and the process is very long and it requires enough vetters and convincing sources and support by more than just a single translator).