This person has a language that makes no sense.
It has a short word for "semantic change." Three of them, actually, distinguishing between unintentional drift, deliberate attempts to change it for the better, and mischievous alterations as an attempt at sowing confusion. Two of those words were used in the conversation these two are having, or rather in the monologue the one in blue is giving to the one in black. It doesn't seem like it's a conversation about the language specifically either, from what Zarian is picking up it's a story about someone's day and an amusing anecdote about standing in line for too long at a sandwich shop. Despite having a word for semantic change, there appears to be no word for sandwich and the one in blue has to use a kind of punning neologism involving the root word for bread, the word for "conjunction" used in a place it grammatically shouldn't be, and the word for zeugmas. What kind of language has more words for parts of speech than it does for common food items?
Oh. That isn't their main language, Zarian can pull a lot of it from the one in blue but the one in black is barely understanding the monologue. Green is trying to teach it to black. If Zarian can pick up languages they aren't currently speaking in she'd notice there's two different languages that they both speak a lot better than they do this one, and a scattering of other weirdly shaped languages they have bits and pieces of.
The gesture is at least straight forward. It's a basic acknowledgement of someone's presence, with a connotation of approval.
The pair is drawing level with Zarian, They don't seem like they're planning to stop hiking unless she does something to indicate they should.