“Intelligible” has a similar meaning to “understandable”, with more focus on the speaker’s ability to be understood rather than the listener’s ability to understand them.
“International” simply means across nations.
Putting the two together, I would expect (without having read the linked article) that it’s about how speakers can use English in a way that is easier for foreign audiences to understand, with a focus in this example on removing idioms that a native speaker may take for granted.
I wouldn’t expect this to be focused on non-native speakers since they’re less likely to know and use English idioms. However, it is certainly possible that they might try to translate idioms in their own language to English, which even native speakers likely wouldn’t understand and, perhaps more importantly, wouldn’t be found in English dictionaries like native English idioms would be, so that may come up as well.
Update after reading the article
I wasn’t too far off, but the lofty title had me expecting a lot more than was actually there. Essentially, the authors took some English idioms and tested whether Japanese English speakers could understand them. The results were divided into three groups:
- Transparent: the students were able to figure out what the idioms meant, even if they hadn’t seen them before.
- Semi-transparent: the students were able to figure them out because Japanese has similar idioms. (This may be why non-natives try to translate idioms: sometimes it works!)
- Opaque: the students couldn’t figure them out.
Such data would support a paper that met my original expectation, but this one doesn’t directly suggest doing anything about the problem.