Eltharin is a highly melodic language, and emphasis on certain words or syllables can eliminate subtleties or intended meanings. But not just that, it's also the sort of language where every word has multiple meanings, and those meanings are often antonyms to one another.
The elven lifespan means that they have all the time in the world to get used to it and grasp the relevant subtleties.
For instance, Yenlui means balance or harmony, but it also means chaos (or possibly Chaos). Sendai means dedication, sacrifice, and also resentment. And my favorite, Tavlu is the rune of Hoeth (god of knowledge), and it also means 'the tower, wisdom, misery'.
So certain complex concepts are just implied into the language, like how dedicating yourself to something or sacrificing your time can make you resentful, or how being wise also means being miserable a lot of the time.
Regardless of speaking it due to a gift, speaking Eltharin perfectly is remarkable.
I think I remember that one of the reasons that elves can master Qhaysh is that they can mentally compartmentalize and have multiple interweaving trains of thought running simultaneously in a way that humans cannot.
Though that may be an idea introduced in later editions, when they were shifting the elves more toward the 40K Eldar model.
Still irrespective of how similar to humans (or otherwise) they mentally are, Eltharin as a language would shape how elves see the world (
as all languages do to varying degrees).
But it could also create problems for someone using magic to auto-translate.
Presumably Elriza is getting everything said in Eltharin translated into her native tongue in her head and/or processed in way that fits how her brain has developed since childhood to understand spoken language?
However, doing it in this matter could mean that a lot of subtext, metaphorical nuance and poetic implication will be lost along the way.
If a sentence spoken in Eltharin can manage to convey what in other languages could take several sentences to exposit; then Elriza is likely getting a very dumbed down version of at least some of what Yavinder might be saying.
Pseudo-audibly translating the additional context in a manner that Elriza's brain is used to processing would take too long in real time and make in person conversation a unnatural, drawn out mess.
In reverse (translating what Elriza is saying into Eltharin), there is no such problem.
However, there may be an issue in how Elriza's speech being translated into Eltharin may result in her saying more than she is aware of.
Her magic automatically expresses metaphorical subtext, poetic implication and varying degrees of obtusely inferred detail in the typical style of perfectly spoken Eltharin.
Elriza might say very little, but unknowingly convey a whole lot.
