Do you setup separate projects per language on a multi-language website?
Will most of your tests be site-wide or will they be langauge specific?
Does your site easily support different look and feel for each region? (is your team built to support different websites per region)
If a site-wide experiment is a winner over-all but flat or slightly negative in one region, will you roll it out globally?
Are you comfortable enough with URL targeting to segment your experiments effecively?
Do you have another analytics tool that you will use to consolidate the experiments from different languages so you can determine an overall impact?
IMO, one project per "code base". This gives the best flexibility and re-usability. But you should consider your team's abilty to support the system in either configuraiton. If your site is identical other than language, use one Optimizely project for all languages and make language available as a segment that you can use when you want to see the results broken down by language.