Old is in Hebrew, New in Greek
both though have some Aramaic.
Jews accept only the Old, not the New. Christians accept both. Muslims believe both are corrupted.
The Old is longer. 39 books, to New's 27, although Jews group the Old into 22, Christians group the Old into 39 books.
The deity of the Old appears more into fire & brimstone justice, while the diety of the New seems more merciful
The Old is more into the Law, the New not so much
They have a red letter edition of the New. But nothing comparable for the Old