#as to whether he'd teach Legolas Doriathrin...I don't know

[imakemywings]

So Doriathrin sounded old-fashioned even in the First Age, which makes sense since Elvish language already changes very slowly and Doriath was comparatively isolated relative to other Elvish settlements (and was already known to have preserved some linguistic characteristics that had become archaic in other dialects). It was also destroyed, which largely prevented much further development of the language owing to the people being made more or less extinct.

Furthermore, since Oropher and Thranduil’s later kingdom was made up of a far larger proportion of Silvan to Iathrim, I tend to think they adapted their speech to that of the Silvan, whether speaking Silvan or adjusting their Sindarin to match with the speaking patterns and idiosyncrasies of the Silvan.

However, in the rare moment Thranduil speaks directly to another former Doriathrim without any one else present in the conversation, he will revert to what he grew up speaking and they will twitter away like they just walked out of Chaucer.