Hola,
I am French and I always adapt my grades. For GCSE I wrote the sujects that I took at that age and adapted the grades. In Frnace we used to get a grade out of 40. So if for example I got 36/40 I write down an A, if it's 31/40 a B etc...
I've always done that for all my jobs applications in the UK, and people know it's different so they are ok with it.
Happier?