Hmm I always thought I speak and write english really well. I have also been told so by a number of americans but then it perhaps has been in comparisment perhaps with other europeans :)
Frankly, your English is imperfect. But, you communicate well enough to get your point across. So, I estimate you are more effective than most non-native English speakers.
In any event, I'll offer you three hints that may help you reach fluency. First, you are less likely to make errors and more likely to be understood if you use simple sentience structures. Second, never use the word such. Third, always capitalize American.
So how should I write:
"I think I have seen such data before"
1: I think I have seen data like that before
2: I think I have seen that kind of data before
I am swedish btw.