It is necessary a Lua function, or alternatively a parser function, basically answering the question "for xx language project does page foo exist?" For use case see https://en.wiktionary.org/wiki/Template:t and https://en.wiktionary.org/wiki/Template:t%2B. In Wiktionaries it is a common practice to add in translation tables an interwiki link to home language project for each translation. Currently, in major projects, a bot changes template:t to template:t+ but this is unaffordable for most projects and now it seems it can be done more effectively throught a query to Cognate.
You mentioned this task during the feedback round about Cognate. Can you explain a bit more the use cases you see for that, meaning: what would you like to do with such a feature? How could it be helpful in your work?
By knowing more about the use cases, we can estimate better the implications of the feature and evaluate better its feasibility.
@Lea_Lacroix_WMDE my main planned use case right now is T190210. Maybe it could also be used to automate the "See also" section at the top of the page (currently maintained manually with Template:also + bot).
Another use case would be to automatically link to other wikis in translation tables (Template:t+).