Ask them all. Why not? The worst thing that can happen is that they say no. The best that can happen is that they all say yes. Then you can choose who you really want. Some letter writers have you jump through ridiculous hoops to get them to write it, so having spares is a good idea.
1. All PIs are musts. So ask your freshman and current PI absolutely. Same goes for the research supervisors for your current lab. There's 4 letters right there...
2. How did the research-oriented course go? Are you close with that prof? That's an option.
3. Foreign lang. prof sounds stronger than the English prof. Which one writes better? That does matter.
4. Other profs / outside recs
I agree with above, but you don't want to ask 10 profs if you're only gonna use 4 letters, you know? Just make sure you're actually considering using their letter if you ask them for one (don't waste their time). Go in the order I listed above when asking. Also make you have at least one non-science rec letter, a lot of schools require one.
You are best off being very direct with every person you ask for a letter: ask whether they think they can write you a strong letter of recommendation for medical school. If the person you are asking seems hesitant, then try your luck elsewhere. I don't think that you ought to restrict yourself to professors in whose classes you got an A - many times professors think highly of students who get B's too
Ask widely and pay attention to the level of enthusiasm/confidence you feel in the room to judge whether the letter will be good. You can also always choose to not include a specific letter even though you asked for it (though that's not nice to the professor in terms of wasting his or her time).
You are particularly in need of two LORs from science profs who have taught you and can discuss your academic prowess. I think that taking Anatomy with the research-course faculty member whom you don't yet know well could result in a much meatier letter than using a freshman science instructor who will barely recognize you. Using the Biochem prof is a good idea too, provided you take care to touch base with him/her frequently through the term (and do well in the class).
I'd also go with the foreign language teacher. If his written English is less than polished, it will not be held against you.
The postdocs from your current lab could potentially write a joint letter that both would sign (and maybe the PI as well).
Many schools will limit the number of LORs you can submit and it would not be wise to ignore a restriction. This would not be the case if your school will submit a committee letter on your behalf, as that would count as one letter, even if they append copies of each letter to the committee summation.