Proficonf provides a language interface at the moment: in English, Russian, Italian, Portuguese Brazil, Spanish.
You can change the interface language in the header in the right corner, in the footer at the right bottom of the landing page (available in Russian and English only).
You can also change the interface language in your personal account settings after login in or signing up (available in English, Russian, Italian, Portuguese Brazil, Spanish).
To change the language of the meeting interface during the event itself go to the conference Settings in the upper right corner, pick "Advanced" and pick the language you prefer. The preference will automatically apply.