Balty specialised in studying primitive societies = some combination of anthropologist- historian.

Note that where languages evolve from a common ancestor (Latin - French, Italian, Spanish, Portuguese Rumanian or Sanskrit - approx a dozen modern languages) a huge amount of the vocab remains in common and so do some of the key grammar rules. This would be accentuated in WoTLand because they never lost the written OT and they continued to trade with each other in general. Not that difficult for AoL-ites then, to pick up descendant language/dialects.

 

Ishamael also had plenty of practice since he was out there for 40-year bursts so, he could give the others a headstart. At least to some extent, Nyn would be able to follow OT perhaps if one guesses that AS teach the OT to Accepted/ novices. (Suian who was a fisherman's daughter knew OT.)

When Rand speaks with Moridin in TGS. Are they speaking the OT? I would think so, considering how intimate their conversation is. I mean Rand is even using Moridins birth name when addressing him. Not even the other Forsaken use the name "Elan" when conversing with him.

Maybe when in the world of dreams language is no issue, you are able to understand as part of the magic of the place

That's probably the most likely thing. Perrin, Nynaeve and Elayne could all understand Birgitte and she could understand them too.