Translation for a Modern Web Application: Gettext
"The domain setting is which group inside of locale_data that the keys will be looked up in."
seems one can switch the domain application wide (so we would have Gyms, Skiing, Hotels as domains) but i'm not 100% sure about that. But it's not very recommended-- see http://stackoverflow.com/questions/9668471/use-multiple-domains-in-gettext-in-a-php-application
For backbone.js and i think other front-end JS frameworks, the best recommendation is: