Translation:Why were you assuming that Czech was an easier language than German?
to assume is usually a attractive been and not used in continuous. It's not impossible bit we prefer 'under the assumption'.
I know it's the aspect etc. It just seems odd.
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?
Czech is not English and does not use English tense agreement rules.
He assumed: "Czech IS easier than German." Present tense. We keep the present tense.
why doesn't the English sentence say is instead of was then?
Because English has different rules about tenses in reported speech. https://www.perfect-english-grammar.com/reported-speech.html
I am not sure actually, but we DO accept the past tense as well. "Žofie asked where we HAD our bathroom." is accepted.