Themes are essentially the reoccuring ideas that pieces of literature expand and develop.
Common themes include:
The struggle of the individual against society or nature
time
death
dreams...
language of literature is any language written or oral in a sense that it adds to more deeper meaning of the passages that was contained in the literary pieces.
it's kind of like the moral or technically the things that make up the book e.g the themes in twilight novel are love, revenge, death e.t.c do you get?