Add preferred language to join form

[wizardishungry]

Porting nycmeshnet/nycmeshnet.github.io#51

We should try to:

• make people whose preferred language is not English feel welcome when visiting our website
• better communicate during installs
• let volunteers build relationships with users and communities through improved communication
• assess our outreach to under-served users

Preferred language should be a free-form field.

I would make a PR but I don't know the Google integration.

[wizardishungry]

It might be useful to include a "notes" textarea as well if a join request wanted to convey other information ("I own this building and want all my tenants to have access").