Is 英語を喋りますか the proper way to ask someone if they can chat online using English?
These are some of the ways I've seen being used(but from japanese people asking if the other person could speak japanese):
英語は大丈夫ですか。- Eigo wa daijoubu desu ka. - Is english ok?
英語で話してもいいですか - Eigo de hanashite mo ii desu ka. - Is it ok if we speak in english?
英語が話せますか。 - Eigo ga hanasemasu ka. - Can you speak english?
As @Felipe_Oliveria has given the way to ask someone about his English speaking capabilities in Usual ways. But you want your words in the proper way then you should use
From "喋ります" to "お喋ります" will make it more polite. Also, the "で" particle is usually used for any Language / Medium. like 英語で、日本語で. Hope this will help you. :)