Some of the discussions have been focused on the trees while missing the forest
Learning medicine is similar to learning another language - full of technicalities, abbreviations, etc. Mastery of the primary language (english) is of utmost importance before one begins the pursuit of medicine.
Anatomy is hard enough with its fast pace information, memorization, latin and greek words, etc. Try learning it through a language filter. If you survive anatomy, there's always histology, biochemistry. embyrology, pharmacology that all have their own terminologies and vocabularies.
If you are struggling with English, you will struggle with these courses.
The USMLE/COMLEX will definately be more complicated (more so than SAT/ACT/MCAT) since they not only require mastery of materials, but also able to comprehend the question (which can be 2nd or 3rd order questions)
Clinical rotations can be brutal. You'll encounter patients of all race, genders, and education backgrounds where you will be challenged to understand what they are saying. Add local (or foreign) accents, along with local idioms and saying and it will complicate matters. You have to be able to quickly and comfortably communicate with your patients, your residents, and your attendings.
Residency will be a whole new challenge (especially with all the paperwork involve).
Some of my classmates whose english weren't their first language struggle with their classes - not because they weren't intelligent (quite the opposite, they were very brillant) but had trouble learning the material through a language barrier.
I suspect your poor grades in the sciences may not be due to your ability to comprehend the material, but your ability to learn the material in English (you have mentioned that you are translating what your professors are saying into your native tongue in your head during class). You have to work to improve your english to the point where it is natural and no translation is needed. At that point, you will be ready to tackle the sciences, the MCAT (and future standardize tests) as well as medical schools.