Can it not be " Ongea Kiswahili tu" ?
Yeah, it can. To say "speak [language]", you can use -ongea, -zungumza or -sema in different regions. In some areas, -sema only means "say".
If you didn't report it, do it if you get this again.
Not sure about it, but shouldn't "Semeni kiswahili tu" be accepted?
Yes, it should.