We have three sense choices for can: pol- "to be physically able" lerta- "to be allowed" ista- "know how to"
The problem is that all of them should be coupled with an infinitive verb for what we're actually able to do. I'll just put them in the infinitive, and assume that's meaningful in the same way it is in the English slogan. I think "ista-" is the best option, since what we can do is more of a collective movement and not a physical act, and no one is setting the rules about what we can or cannot do.
The we suffix -lmë is inclusive (both the speaker and the listener are in the we, as opposed to exclusive -mmë; English doesn't really make the distinction).
Finally, yé is an untranslated positive interjection that seems to fit the feel of the slogan.
"yé! istalmë!" -> "yeh ihstahlmeh" (only the é is a long vowel, as in German "mehr"; the others are short vowels, i as in "pit", a as in Spanish "padre", ë as in "end").
So, tlhingan Hol is not generally a positive language :o). A bigger issue is that ability as a verbal concept doesn't exist as a standalone; it's always just a verbal suffix. What particular sense of "can" do you want?
I'm going to go for a particularly Klingon sense: "Yes! We can defeat them!"
The pronomial prefix for subject we, them as an object, is DI-. The root verb for defeat is jey. The verbal suffix (type 5) for ability is -laH. The interjection "yes!" is "HIja'", which you've probably heard in various Trek sources.
H is an unvoiced velar fricative (as in Bach), ' is a glottal stop, I is i as in "pit", ey as in "hey", D is a retroflex voiced dental stop.
I just looked roots up in the references and made some word sense choices, and butched some pronunciation, so you should probably give more credit to:
Incidentally, a lot of Klingon's "weirdness" comes from Okrand being an expert in the Athabascan amerindian languages of the western US; while he picked a very guttural phonetic library, a lot of the structure is borrowed from his study of those (now mostly moribund or extinct) languages. Similarly, Tolkien-the-linguist liked certain features of Finnish and made sure to include them in Quenya.