This is a chicken and egg problem, assuming you have not imported subtitle files.
MC has your video file and its name:
Director - Title - Year.avi
There is no LANG component. But the trick to getting MC to ask about a file NOT in its library is to derive a name from what you have, and ask MC if the file exists:
if(!IsMissing(Director - Title - Year.ENG.srt), ....
Note: I'm ignoring how we create that name above, as the expression is pretty easy.
Well, since you've specifically asked MC about the ENG subtitle file, you know already that the language would be ENG! So the expression to use to assign to your language field would be:
=if(!IsMissing(Director - Title - Year.ENG.srt), ENG,,)
That's not terribly useful by itself, since you could just assign ENG to your language field (so long as a subtitle file exists). To be more useful, you'd have to make a more complex expression, testing for different languages, each sub-expression returning the language value. This would allow you to select a bunch of video files and do a bulk assignment. This is what you wanted from File Properties from Filename. Pseudocode ahead:
=ifelse(!IsMissing(Director - Title - Year.ENG.srt), ENG,
!IsMissing(Director - Title - Year.GR.srt), GR,
...
)
The more tests you add, the slower the performance because MC's IsMissing() function has to check the file system for each and every file, each time it is used.