As the primary goal of an ERASMUS period of study abroad is to study at the host university, not just to learn the language spoken in the country, a good level of proficiency in that language is required. You should be sufficiently proficient in the language spoken in the host country to be able to settle into daily life there. This is the case even if the university offers courses in English.
The Language Centre (Zentrale Sprachlabor, ZSL) at Heidelberg University offers courses in different European languages every semester. You can prove your level of language proficiency by taking a language test at the Language Centre (form available from room 139 in the International Relations Office). Should these courses be full, you may also complete language courses offered by other language schools (e.g. at adult education centres, VHS) Please note that to be awarded an ERASMUS scholarship, written proof of a sufficient level of language proficiency is usually required.
Many host institutions organise intensive language courses, which take place before the beginning of lectures, in the language spoken in the host country. Information on these (and also on possible financial support), is available from the International Relations Offices at the respective host institutions.
Online-courses are available through the OLS plattform. Licences for the OLS online-language-course can be provided by the International Relations Office in Heidelberg.