I am trying to read Mumford's Geometric Invariant Theory, however, I find my knowledge in algebraic geometry is inadequate. My knowledge is at the level of Hartshorne's Algebraic Geometry. Mumford cites a lot of results from EGA and SGA, but I cannot read French. Therefore, I want to ask:
What should I read if I want to read GIT?
I should mention that most of my knowledge is in differential geometry. I want to read Mumford's work to understand stable bundles and moment maps from another perspective; different from differential geometry.
Any suggestions for alternatives to Mumford's work will also be acceptable.