I would like to know how the UK equivalent/translation for a French student who is BAC+5 and going to get his engineering degree. Is it under/post/graduate?
A potentially useful article for you would be the Wikipedia page for National Qualifications Framework. The UK education system categorizes qualifications in 8 'levels'. The BAC + 5 is level 7. If you have a Masters in the UK is it assumed you completed previous study and attained at a bachelor's degree (or equivalent) so it is offered as a postgraduate degree. Masters programs are also known as 'graduate' degrees which is an import from US English.
Whether or not a student is an undergraduate or postgraduate depends on whether they have already got a degree. Someone working towards their first degree is an undergraduate. Someone who already has a degree and is working towards a higher degree is a postgraduate.
I don't know if 'is BAC+5' means this person already has a degree or not but presumably you can work out the right word from the above.
For what it's worth: In America we normally talk about four "levels" of degree: associate, bachelor, master, and doctor. When you don't yet have a bachelors, you are said to be "undergraduate". An associates degree typically requires 2 years of study, and each higher level degree 2 more years over the previous.