1 - You shouldn't re-use a name that already exists. Elm is a long existing mail client. Pick a different name.
Doesn't matter. Elm is a unique name for a programming language. That something completely different exists with the same name doesn't make any difference to either project.
Besides, if Google can name a language Go, even though a programming language was already named Go!, then naming a programming language Elm, even though an email client is already called Elm, isn't nearly as confusing.