(I don't know whether it is proper to post such a problem here.)

I am trying to read Mumford's Geometric Invariant Theory, however I find my knowledge in algebraic geometry is inadequate. My knowledge in ag is at the level of Hartshorne's Algebraic Geometry. Mumford cited a lot of results from EGA and SGA, but I cannot read French. Therefore, I want to ask that: What should I read if I want to read this monograph?

I should mention that most of my knowledge is in Differential Geometry, and I want to read Mumford's work to get a comprehension in stable bundles and moment map from another respect, which is different from differential geometry. Any alternatives of Mumford's work will also be acceptable.

I am not a native English speaker, so there maybe some mistakes above. I would like to thank anyone who is willing to help.