Hmm, a lot of Czech words aren't actually Czech, then. I think that makes even less sense. The fact is the Czech language loans heavily from many different languages (Latin being one of the top donors) - but that doesn't mean we speak a mix of languages in one sentence.
The prefix de- is not used only in loanwords, you can construct new words with it just fine. It doesn't sound right in your example but that doesn't mean it's nonsense.
The prefix de- is not used only in loanwords, you can construct new words with it just fine. It doesn't sound right in your example but that doesn't mean it's nonsense.