"Cockroaches cover the vegetation!"
Translation:tI luvel vetlhmey!
Yes. All letters in Klingon are either upper case or lower case and only the Q and the H occur as both, but they are not examples of different cases of one letter, but separate letters. Thus the first letter of a sentence should be capitalized or not depending on the letter and not the position. The Duolingo software was not designed to account for this and it sometimes recapitalizes inappropriately. There is no way to fix this without major reworking of the software. Supposedly it is on the to-do list of the programmers, but it is considered a low-priority and high-resource project, so is not likely to be fixed any time soon.