Generally you can write your cards in any language.
You can also edit the reply options on the reply page to any language.
The standard text of the Event/Sending is settable to 3 languages at the moment: English, German and French.
If you are therefore sending Spanish, Russian or Chinese recipients your invitation, we would recommend to write the card in the language of choice and use English as the "event language".
You can find the LANGUAGE SETTING button per event in the top left corner just under the tab DESIGN.