Why isn't it "I know" since umim's hint is "I know", wouldn't "I speak" be mluvim?
I translated the sentence in the same way as Mr Bass and agreed with his analysis but was marked wrong. An explanation of why would be most useful.
Curious. "I speak Czech the best here" (my answer referenced in the earlier comment) IS among the acceptable translations, as is "I speak Czech best here." Maybe there was something else in your answer that Duo didn't like. (That's why it's always helpful if we copy/paste our complete answers into our comments.)