You have to provide a language certificate only if the language of instruction at the host university is different from your mother tongue. If this is the case, this document has to be issued by an official language school, university or equivalent and state the level of proficiency you hold in that language.
In case the language of instruction at the host university is your mother tongue, then you will have to write a letter yourself stating that condition. This could be an example:

"I, (your name) hereby confirm that (your mother tongue) is my mother tongue”
*Don’t forget to include your signature and date.
**Remark: Each program/school/university has its own language requirements (TOEFL, DELE, etc). Each course/program offered includes information about specific language requirements. In case you do not have your TOEFL, DELE (or similar test results) yet, you still have to upload an official language certificate in the language of instruction at the host university. Keep in mind that, if you are granted an Erasmus Mundus – Al-Idrisi scholarship, you will have to provide the results of your language test before starting the mobility.