How Xeyr (etc)'s language powers work

Xeyr (as well as her companions) can generally use and understand any language someone they're trying to communicate in language with may be using. This generally works in the following ways:

• It is constrained by physical ability. If they are around people who for instance use a sign language that requires three hands, or that uses the modality of a flashing light on the forehead, etc, this won't give them extra hands or forehead lights or anything that would compensate. They will know how they would express something, but they can be unable to physically do it. One the other side, if they're around people who for instance communicate by projecting magic auras that only people with that magic can perceive, this won't give them the ability to perceive it. If they perceive it, they will understand it, but they don't get any advantage in the perception.
  
  
• If they are trying to communicate something in a language, they will be able to think of how to communicate it in that language, the same way someone who regularly knew that language would. This includes noticing the lack of a direct way to express something. (At which point they can try to talk around it, explain it, etc). They retain whatever level of eloquence, literary talent, etc they usually have and neither gain nor lose any. They do not gain any inspiration from that fact that some concepts exist in a language unless and until they try to express something that uses those concepts, or to mentally explore the concept area. Their fluency includes things like units of measure that people who speak that language use. 
  
  
• The general rule for comprehension is that when it comes to words/lexical items, the only information they gain through their powers is about the mapping from words/lexical items to mental concepts. They do not gain any other knowledge they did not already have. If they perceive something in a language, they will fluently understand things like sentence structure, grammatical meaning, etc. It is not the case that they necessarily understand all words/lexical items.
  
  In general they will understand a word/lexical item to the extent that it maps to concepts that they already know or can conceptualize. This does not mean only things they have already heard of or that other languages they've known have words for. For example, if they perceive a word that means 'houses with red roofs in particular' they will understand this, even if they've never encountered a language that had a word for that specifically. If they perceive a word that means 'a rabbit with wings', knowing what a rabbit is and what wings are is enough that they will understand this, even if they'd never encountered rabbits with wings before or known they existed or thought about the idea fictionally. It is, in specific, sufficient that they could have gone 'well how about a rabbit with wings', given knowledge they have.
  
  However, if they perceive a word that means 'houses that share a specific feature common in a local architectural school', and they don't have knowledge of these schools, and couldn't identify the feature and what counted and what didn't, they will not gain or acquire that knowledge by hearing the word. They will understand that the word refers to a subcategory of houses, and has to do with architecture (assuming they know what houses are and what architecture is); they will know that it has more meaning that they do not currently understand.
  
  This works with general knowledge the same way it does with local knowledge. If one of them perceives a word that means 'integral' (the calculus term), and they don't at that time know calculus, they will not gain any calculus knowledge by hearing it. They will know that it has to do with math, and that they don't know/understand more than that. (If they have more partial knowledge they get more understanding - if they know that calculus has some parts that deal with rates of change and some that deal with areas under curves, they will know that 'integrals' have to do with the latter). In these cases, they would have to have the relevant knowledge concepts explained to them separately in order to understand the full meaning of the word.