They already can ;)

I was not being sarcastic at all—I don't see English offered as a language—but it's the base for all the others, is that it? Because I start from English, I have less insight into the user experience of someone who begins with another language, especially one not offered here.

As far as I know, there are English trees for people whose proficient language is Spanish, Portuguese, and Italian. When you start Duolingo and indicate that your language is English, you can't access the English lessons!

You can learn English from Portuguese, Italian, Spanish, and French :) If you speak English already you can learn Portuguese, French, Spanish, Italian, and German. You can see all the language options in your settings under 'Learning Language' too.

Thanks—got it (duh!)