Could you provide an example of a template? As someone who isn't an English Wikipedia editor, I don't know what exactly "in a citation template" would be referring to. What currently happens when you try to use ruo?

Since you seem to want an English name for the language, I assume the template is using the {{#language:...}} parser function (which uses data from CLDR). If so, CLDR doesn't include ruo, so the name would need to be added to LocalNamesEn.php in the CLDR extension.

This is what I mean. Putting the parameter "|language=ruo" produces "(in ruo)" instead of "(in Istro-Romanian)" as you can see on reference number 50 on this English Wikipedia article [https://en.wikipedia.org/w/index.php?diff=1171829642&oldid=1168374394&title=Istro-Romanians] (ignore the publisher/magazine thing). However, if I change the value on the parameter to for example "=ro" [https://en.wikipedia.org/w/index.php?diff=1171829664&oldid=1171829642&title=Istro-Romanians], it produces "(in Romanian)" without any issue. That means a certain something includes Romanian but not Istro-Romanian in its code, I assume.

The template in this case would be Template:Cite book, however as far as I know Wikipedia's citation templates (others like Template:Cite web) are all connected and share code up to a certain point, which in the case of language codes might as well be what you just mentioned.

It would be great if Istro-Romanian could be included there. Is this possible?

Change 985398 had a related patch set uploaded (by Nikki; author: Nikki):

[mediawiki/extensions/cldr@master] Add various English names that have been requested

https://gerrit.wikimedia.org/r/985398

Change 985398 merged by jenkins-bot:

[mediawiki/extensions/cldr@master] Add various English names that have been requested

https://gerrit.wikimedia.org/r/985398

It has been added, thank you! My only issue is that the hyphen has not been included (I think it was already like this, as the language was not missing from all code of MediaWiki). Thus, instead of "Istro-Romanian", "Istro Romanian" shows up. I have opened another task (https://phabricator.wikimedia.org/T357981), maybe it could be done at the same time?