The OAuth applicaton approval process on meta.wikimedia.org is probably going to work similarly to the bot approval process, so the owner needs to create an entry or subpage somewhere. The success message after they register their consumer should warn about that. This is installation-specific so the software part is that part of the success message should be untranslatable, and then the message needs to be created on some meta MediaWiki: page. (Or if an English interface message can override non-English ones, no software change is needed at all. See T57473.)
At some point it would be nice to fully automate this.
See also: T67750: OAuth consumers should be automatically approved (will require a different message), T61772: Add notification to inform oauthadmins that a new consumer request is waiting