As a non native English speaker I am more often solving English issues than Czech here :)
Why "My colleague spoke yesterday with that artist" is wrong ?...
See, for example, https://www.ef.com/english-resources/english-grammar/adverbs-time/
"My colleague Matej spoke yesterday with that artist". should be accepted.
Why can't this be "was speaking"? Isn't mluvit imperfective?
I will add options that use "was speaking" to the acceptable translations, since the verb used here is imperfective... if one of the CZ natives on the team will confirm that this would not be a problem.
I think it is fine, I just wasn't sure about the English side, whether the interval does not have to be more concrete or something like that.
Thanks; alternatives with "speaking" and "talking" have been added.
Why "My colleague Matěj was speaking with that artist yesterday." is not correct?