What is the best way to create a custom language file?

Hi guys,

I would like to create my own version of the language files (cleaning up some of the grammar errors, changing the text if the error messages to something that I like), what is the best way to go about that?

I can base mine off the original one from SI, but am concerned about keeping my translation file in sync, what can anyone suggest?

I will gladly share my modifications with the community.