State primary schools in the UK - Europa included - are not allowed to select students based on ability or background, and that includes the language ability/ having a French/German/Spanish speaking parent. That would be simply unlawful, and I am very certain Europa do not do that. In fact the Admissions are managed by Oxfordshire Council including into higher years except for 6th form.
As for the bilingual IB diploma, I do mention it because that’s ultimately the only reason why parents choose this school, hence important to know that only 19 students out of 90 pupil intake achieved it in 2022 after attending this school. To be very honest, when you do a standard A-level in any foreign language and get a B, you’d be also bilingual. I know this because I myself teach MFL and typically my students get 7-9 at GCSE and A* to B at A level.
In any case, good luck with it. As I said, I believe it’s a good school if you live nearby, but you have to keep an eye on your child’s qualifications and avoid an all too common scenario at this school of kids leaving without any qualifications at all or with 2 x GCSEs. To be honest even if they end up getting IB, they will have lifetime problems with just 2 x GCSEs, because in this country you need GCSEs to enter any postgraduate professional training including teaching for instance. It is therefore an educational experiment you’d be part of.
When I said that I know people who went to this school, I mean actual students who have long graduated from the European School when it was still run by the European Commission. A lot of the same teachers are still there. The situation with qualifications - albeit it was the European baccalaureate rather than the IB back then - was the same. Always. Including the low ratio of graduates compared to the intake.