To my knowledge, "Translate" is a verb and not a noun. So why does the above advertisement by Google uses a verb? I mean why not "Translation Community"?
"Translate" here refers to the name of the product: Google Translate. This makes it a noun-adjunct, I believe. You can see them use the full name elsewhere, when they have more room (emphasis added):
You can help make our translations better, and even add new languages, as part of the Google Translate Community.
Although Laurel's answer has been accepted, I'd like to take a different angle.
The banner on Google's site reads:
JOIN THE TRANSLATE COMMUNITY
Because the banner is on Google's own site it would stand to reason they're talking about their own product. Thus, you can drop the "Google" in Google translate as a way of contraction. You'd still be able to recognize the "Translate" as a product name because it's a proper noun, capitalizing the first letter. The confusion stems from the sentence being all uppercase.
If the sentence wasn't all capitals, the intent would be much clearer:
Join the Translate community
Its a product name so doesn't have to follow grammatical rules. Like Donuts is a stylised spelling of doughnuts - thanks to Dunkin’ Donuts, even Facebook, which began as The Facebook Without "the" it seems cleaner and simpler. The same would be true with Google Translation without the "tion" seems cleaner, more snappy.
You can name a product anything you like; it can feature unusual uses, bozo collocations, unorthodox orthography, nonce words, nonsense words; it can even take something else's name, if it's in a different class of goods or services In short there are no rules for products, just the creativity of branding to make an impact.