I don't think "a language I don't use" is a big concern in these email, just like I don't think I should be tested on a specific language in job interviews. It's always a plus if I know the technical / implementation details of the platform, but if I know how to code and I know how to learn languages it's mostly moot, IMO. However I definitely hate it when people pitch me a job that's a "perfect match" for my background and they clearly have no idea what my background is. Don't be disingenuous. If you think it might be up my alley, just tell me about it.