Maglor : lol so Mags speaks by DEFAULT Feanorian Quenya (with the 'th') but he is perfectly capable of switching to 'standard' Quenya, he just won't because fambly. He also speaks Sindarin fluently and these days probably mostly thinks in it. He also has Taliska (the original human tongue) and probably proto Easterling (thanks Bor)
depending on how long you think he lives... if he makes it to the Third Age he almost certainly proceeded to pick up: Adunaic & Westron for sure. Maybe Rohirric, and if he keeps going past the Third into the Fourth, he'll probably get around to Khandian & Haradric
Erestor: Speaks both Feanorian & standard Quenya and will code switch depending on who he is talking to (if he wants to Annoy People he uses the thorn), but grew up speaking Sindarin and prefers it. Also speaks and reads fluent Adunaic, Westron & Taliska.
Elros: Speaks, by default, Sindarin. HOWEVER. He can and will use Feanorian Quenya if he thinks it will be funny, or he wants to make a Point. Also speaks Taliska, but hilariously does NOT speak Adunaic, as it evolved after his time.
i am entirely entertained by the fact that feanor's pet grammar quirk means that he technically speaks more like his siblings' relatives than well, fingolfin does
mags teaching babby bri some choice quenya when he accidentally swears near herlol yesgood au where he babysits her regularlyalmost no one else gets her to bed on timecrack AU where he steals into Lothlorien to babysit her, just to spike her momyesssssssssssCOUSIN GIVE ME BACK MY BABYBri just gleefully climbing him like a tree"Mine. My uncle Maglor. I'm keeping him."I think she likes me best, cousin~SHE LIKES YOU BEST BECAUSE YOU SPOIL HERI mean he probably did that before anywaysFëanor,stop trying to make fetch happen, it’s not going to happenwell, fingolfindoes