Although they
ask candidates for BBB grades (assuming they're not contextual candidates), the typical Modern Languages and Linguistics student at the University of Essex has BCC grades (
source) which suggests that your CCC grades as a contextual candidate may well be acceptable.
As you will know from having read about the course, you choose two languages to study - one of which (the main language you study) you study to "mastery" level (their word). I have two questions. (1) Are you planning to choose Portuguese as this major language? (2) You say "I can speak my own foreign language", but without mentioning what that language
is. Is it Italian, French, Romanian or Spanish? If the answer to (1) is Yes, then the answer to (2) should be Yes (or you should have studied the specified languages at A level). If the answer to (1) is No, then the answer to (2) doesn't matter.
Finally, you say "I hope University of Essex can reply to this". If you want a reply from the university, you should
contact the university. Positing a message on TSR, in the general "Applications, Clearing and UCAS" forum, and just hoping the someone from University of Essex stumbles across it, isn't really the best approach.
(Once I post this reply, I'll move the thread into the University of Essex forum, which might help.)
Edited to add that other that for Portuguese, there is no specific requirement with respect to your level of knowledge of any particular non-English language. So the "basic understanding" you refer to should be fine.