Translation:Are we Klingons? Yes. We are Klingons!
Grammatically, the plural suffixes are not required in Klingon. tlhInganpu' maH and tlhIngan maH mean exactly the same thing.
There are some sentences where it could be confusing. If a pair of people say SuvwI' HoD je maH, does that mean that one of them is a warrior and one of them is a captain or that both of them are warriors and captains? Without more context, you can not be sure. Though if they say SuvwI'pu' HoDpu' je maH then it makes it clear that both of them must be both of those things.
Grammatically, the plural suffixes are always optional, but for good style, you should use them if context or other parts of the sentence (like a prefix or a pronoun) don't make them obvious.
Or the suffix -pu' is optional on tlhingan?
From the tips and notes to the "Basic" (aka "Sentences-1") lesson:
Plural suffixes in Klingon are optional and can be left off. Later on, you may come across sentences where only the grammar tells you that a noun must be plural.