It's good to see you on here Rytis.
Yes ... OTIS
versus Ansur - is that really progress? ...
Unfortunately this sort of thing is not unknown when small pioneering companies get swallowed up by the Big Boys!

But there are two different "back stories" there, of course;
viz Bio-Tek, and Metron.
Which CMMS do you use (that's compatible with Ansur) by the way?
And which Ansur version are you using (and on top of which flavour of Windows)?
OK ... the famous LNG files. As you know these are simply ASCII text files where label strings can be edited and substituted for another. But (I'm guessing here) that Ansur itself will only be able to display characters within whichever character set it uses. Have you tried other font character sets?
I don't have Ansur here just now, so can't check myself; but what I would try is to enter some label changes in a couple of LNG files using a font character set that includes the characters you want, save (using, for example, MS Notepad) with Unicode encoding ... and see what Ansur displays. No doubt you have already tried this.

Meanwhile, I suspect that Fluke have misunderstood your requirements - which I presume to be that you wish to customize the Ansur software to show Lithuanian wording in the menus, labels and button captions
etc. ... as well as when entering notes in the text editors - am I on the right track here?
There's no "firmware" involved there - they must have thought you wanted Fluke test equipment to display Lithuanian legends (menus
etc.) (... and why not)!
Lastly, in MS Notepad terms, which script does your language require ... am I correct in assuming "Baltic"?