Language skills
Your knowledge of different languages certainly helps in getting selected for studying abroad. If you are planning on going to a non-English speaking country, it's worth it to look into language studying opportunities at e.g. the Language Centre or Erasmus Intensive Language Courses (EILC) which are offered to students accepted for an Erasmus exchange. EILC courses are usually in session before the start of the universities' semester. Students accepted for an Erasmus exchange should apply to EILC course through home university, University of Turku. The EILC application can be found here.
One way to show your language skills is to create an Europass profile. You should check with the individual receiving institution about language requirements and certification of language skills.
Language tests
Generally speaking, Erasmus exchange does not require proof of language skills. The receiving institution will inform you of what is needed from you concerning language requirements. If certification of language skills is required, it usually concerns English-language skills. If this certification is needed for your application and if your native language is not English, you may use the scores used in your Master's degree programme application for verification of your English-language skills. Native speakers of English are exempt from English-language certification.