Well, Tolkien used Old English as a stand-in for 'real' Rohirric in the books, and we have very few genuine Rohirric-words.
Regarding Nellas; I would think it's Doriathrin, a diaect of Sindarin spoken in (you guessed it) Doriath. The last part,
-las, is probably the same as the
-las in Legolas meaning leaf. (Legolas being a 'Silvanized' form of Sindarin Laegolas, 'green-leaf'.) The first part may come from
neldor, "beech". So perhaps the name means 'beech-leaf'. Or perhaps not.