English is very, very old version of our language. They do not so much always sound the same.
I love words! I teach myself -- many books and papers float down the river, see? I collect, I learn them. We do not write, so it not so easy always, but we learn English to fight the Mountain Men who were here before. Only combat, mostly, but it make for good base for knowing.
Yes, we have the same language amongst clans for use, but each clan also have separate ways to speak. Most clans are unite under Commander Lexa, so one tongue is good.