Someone at Sterc made an effort (some while ago) to make the extra compatible with MODX 3 (see this PR). But the current version has bugs (see this topic) and I don’t know if there are plans to fix these or if they gave up. (You could try to contact them to see what the current situation is).
In general, the Lingua extra hasn’t been updated in almost 10 years. I think it’s fair to assume, that there won’t be any updates anymore from the original author.
As PHP (and MODX) progresses, sooner or later the code will break.
Someone has to be willing to take on the project and maintain it.
Personally I’m of the opinion that multiple contexts (and the extra Babel) are better suited for MODX:
The normal MODX resource caching can be used (I believe with Lingua the page is mostly uncached, which is bad for performance) and for example search extras like SimpleSearch work out of the box.
There have been other attemps (by the russian MODX community) to accomplish something similar to what Lingua does.
There is the extra Localizator. But it’s seemingly also not compatible with MODX 3 (see this issue).
And then there’s the paid extra multiLingual. (But I have no knowledge if this project is any good and if it’s compatible with MODX 3).
It’s a real shame, because for the customer, it was much simpler and more comfortable than having the impression of having two different sites (or even more, depending on the number of languages). Particularly when there’s a lot of data, and you have to replicate changes in each context, it’s really more constraining.
Switching this site to MODX 3 is going to be hell…