I realize that you mainly mention translating grades between Spain-UK-USA but the final statement of your question is about converting grades in general, so that's what I will refer to.
Most countries have their own system which doesn't really make sense as you try to "translate" them to some other system.
For example in Sweden two grading systems exist at university level:
- U/G/VG which stands for fail/pass/pass with distinction
- U/3/4/5 which stands for fail/pass with grade 3/4/5 (5 being the best possible grade) used primarily in technical (such as engineering) universities/faculties
Our neighbouring Denmark however seems to have a 7-grade system with a scale from -3 to 12. In Germany, however, the best grade you can get is a 1, thus you would want a "GPA" as low as possible. So much so that if you apply to an institute with 4+ GPA from a Swedish engineering school, they don't even bother to reply (personal experience).
To add to this grading madness, consider the effect of the curve bell; in some countries university exams are graded based on a bell curve, so a certain percentage of students get the best grade no matter how many points they get. In other words if you are in a class of overambitious students, well, your 80% in the exam might be barely enough for a pass.
Note that a standardised grading system was attempted in the EU (with some extra countries) in connection with the Bologna Process, with the ultimate goal of facilitating internationalization amongst European countries. But it died out due to the resistance from many different institutes. When the standardised grading system introduced with the Bologna Process was trashed later on (at least in Sweden), the solution was to supply a diploma supplement, for instance upon enrolment in exchange program, where statistics (how many students were enrolled, how many passed with what grade etc) for each course the student has taken is denoted.
To sum it up, what the grades are supposed to reflect typically gets lost in translation. Trying to convert them back and forth does not make any sense, and even if it did, there is no guarantee that country A and country B will value a certain grade G from a country C the same way. In clearer terms your 6.7 Spanish GPA might not weigh equal when judged by the American and the British authorities/companies.