this is a work in progress
Where should I start?
Start reading the Area 51 Help.
Then Internationalization 'State of the Stack'
If you didn't read the above linked items, before continue reading this answer, read them!!
How long will take to have a site in language
<x> up and running?
If there is already a Stack Overflow site in the
<x> language it will take around ___.
If not, it could take several years, so in the meantime the best that you can do is to support the proposal of Stack Overflow in language
From Internationalization 'State of the Stack' - Stack Overflow edition
If you propose something and it's closed, it's not us saying no. It's
simply us saying not yet, and probably not for a while, but we're
working on it.
How can we get people that don't speak English participate?
Definition and commitment phases
For the definition and commitment phases as the Area 51 UI language is English you should refer those people to Area 51 translated guides or tutorials and in they are not available yet, you should consider to create some of them. An alternative is to try yourself the use of automatic website translators and if they work fine enough recommend to use them.
Bear in mind that sample questions could be posted in the target language, but discussions regarding the proposal should be posted in English and could include a translation to the target language.
- Lee las Preguntas Frecuentes de Area51 en Español - Read the Area51 FAQ in Spanish
For the beta phase most of the site will be already translated so you could focus on promoting the site and encourage people to use related meta site to report any problem with the translation of the UI in the same way of any other problem or concern.