Hi zhongjiukuai,
there are about three ways to do this:
1. I tried running the flashcards text file through Google Translate, which would be the more error-free way because there is probably some intelligent algorithm at work that looks at the whole sentences. Too bad it only converts about the first 10 KB, after that the text file stays Simplified. Perhaps one would have to get a paid Google Cloud Platform subscription to convert files of any size.
2. Run it through Pleco. To accomplish this, all you need to do is switch the first and the second columns around and then choose Add Traditional headwords in Batch and switch the columns around again. Excel, LibreOffice Calc or another spreadsheet allows you to do this very easily. Then the quality of the Traditional text is quite good, though there are some errors because the algorithm is word-based, and Pleco wasn't really designed for converting entire sentences between Traditional and Simplified.
3. Use a free Python package, which unfortunately produces the worst quality results because individual characters are just being switched between Simplified and Traditional.
We can leave out 1 and 3, so 2 remains. I just tried number 2 with the file I had posted at the beginning of this thread. It works fine. Perhaps could you send me the file you'd like me to convert, as I sense you are interested in a different file. Or you could try it yourself using the method described above.
Cheers,
Shun