There is a language certificate that has been specifically developed for the Direktaustausch-program with Latin American universities. It's not the same as the DAAD certificate. This language certificate needs to be sent when applying and can be done at the Freie Universität's Sprachenzentrum. There are fixed dates every semester to do the exam in order to get the certificate. It is free of charge. However, you can also use an official language certificate from the Instituto Cervantes as well as proof of native language (it has to be clear in your CV that you lived and grew up in a country where the language is spoken). Proof of language skills through a study program such as Romance studies or Latin American Studies can not replace the language certificate and won't be accepted.