I'm not entirely sure what you are trying to express, can you break the individual thoughts down? Then I can help you.

-I think, in the work,one of the most important things is to use english to document things, which I have done so much in my thesis, so i can handle it. or -I think, in the work, one of the opportunities to use english is to document, which I have done so much in my thesis, so i can handle it. It is kind of difficult to figure out what you mean, but I hope this two examples I give you, help you out!! Have a great time!!