"August is a good month."
Translation:Sierpień to dobry miesiąc.
Yes, as generally many ungrammatical sentences in any language could be understood... but it doesn't change the fact that "Sierpień jest dobry miesiąc" is not a correct sentence.
Almost, you missed a letter. Well, and the special characters, but that only counts as a typo (even though they're in fact different letters...)
So it's "Sierpień jest dobrym miesiącem". The whole noun phrase "dobry miesiąc" takes Instrumental.