How can I offer my members two or more languages on my wordpress?

Dear WPMU Team, thanks for the last answer. I used Poedit and translated one plugin. I did not set my wordpress on one specific language yet bc I want to offer two languages. EN & DE.
Could you pls let me know what I have to do to give my visitors and members the ability to choose their preferred language.

Is there a plugin for that?

Thank you very much
with best regards
Ratschi