When you click on ''Add links'' to add interlanguage links on a cawiki article, it does not allow to link to enwiki, for example, and you're able to choose only, for example, enwikibooks.
This happens randomly: sometimes it happens and sometimes, it doesn't. It also happens in cawiktionary.
The workaround is to link from the equivalent page in another language or from wikidata itself.
Last discussion with links links to three other discussions about the same topic: https://ca.wikipedia.org/wiki/Tema:Vinajutl5v4dd3oo