Gith languages

I think it was the Time of the Dragon boxed set (for Dragonlance) that introduced the idea of using percentages to measure how closely related different languages were. So Italian and Spanish might be 75% mutually comprehensible, while English and Spanish only 25% (to pick some fairly arbitrary numbers).

You might add a DC for people who speak a certain language to be able to understand things understood in a closely related language. So understanding Githyanki for a Githzerai speaker might be relatively easy, but certain words or phrases might be garbled or obscure, especially considering the habit of githzerai speaking in metaphors that require knowledge of their cultural heroes to decipher.

Metaphor and Context are the two major distinguishers.

As pointed out by others above, the events surrounding heroes, villains and major landmarks in time that led to the use of certain phrases would result in differences in understanding. Whilst an expression used by a Githzerai might make sense in terms of the actual words used for a Githyanki - if that Githyanki lacks the knowledge of what that metaphor refers to then it becomes indecipherable.

Here a real-world example: try translating the phrase 'once in a blue moon' in to another language. I've done in several and it made no sense what so ever to the native speakers.

The other thing to bear in mind is that the Githyanki and Githzerai live in very different realities (i.e. planes). The Githyanki, I would guess, have a very different perception of time and space than the Githzerai due to living for many generations on the Astral plane. The same goes for the Githzerai in terms of surviving in the chaos of Limbo.

As seen in the Planescape: Torment game, the Githzerai place a great deal of value on self-knowledge. The Githyanki have a mind-set and way of looking at the Multiverse that has been shaped in part by living on the Astral.

That perception is likely to influence the way in which they use their language. The way in which a language is used also has an influence to some extent on the way that they think and this forms a feedback loop with the above.

So whilst the Githyanki and Githzerai have similar words that they use in the everyday use. For one to understand the other, in my opinion, would require a broad range of understanding of the other's culture & history - as well as trying to think like the other side. I don't know how many Githzerai would want to think like Githyanki and vice versa.