Translation:Why were you assuming that Czech was an easier language than German?
Both are accepted. Both seem to be possible https://books.google.com/ngrams/graph?content=why+do+you+assume%2C+why+are+you+assuming
BTW, I do not want to sound sarcastic and I take your comment 100% seriously, but try to avoid typos and spelling mistakes when making arguments about grammar, it can make you look less credible than you might be. If you make a typo, you can correct it by editing the post.
As a native (US) English speaker, I can say that "assuming" is used quite often in the US, in the continuous form in multiple tenses. I am assuming -- I was assuming -- I have been assuming -- I had been assuming -- I will be assuming.... all good.
While "under the assumption" is definitely also used, I'd say it's probably used less often than the various "assuming" constructions. Perhaps there's a significant difference in US/UK usage?