Don't allow members to speak in multiple languages within the same community.
The reason should be obvious.
Fewer members will be able to understand it. Every foreign-language discussion is the same as spam to a non-speaker.
Imagine you have an audience that includes speakers from English, French, and Spanish speakers. All things being equal, members will only be able to understand and participate in 1/3rd of the discussions.
However, all things aren't equal. You will usually find 75% of the audience speak English (first or second language). You will usually find one group has significantly more or significantly less interest in the community.
Don't try to please everybody. Focus on a slice of your audience that has as much in common as possible. Build one community at a time. Once you have one community going, then consider creating another for a different language.