I think that the fewer the number of speakers of a particular language, the more likely are they to have a high proficiency of English. I don't know any Polish people, but where I live, in the Netherlands, people speak fairly good English. Most of them have quite a thick "foreign" accent, and most people, even academics make quite a number of mistakes, pronunciation wise, but also grammar wise. On the whole, it's quite decent, compared to let's say native speakers of Japanese, Chinese or French, who, for the most part, speak very little English.
Edit: I actually know one Pole (not personally), Successful English learners: Tomasz P. Szynalski | Antimoon. His American accent is flawless.
Nganasan aren't profient English speakers. There are more native German speakers than native Polish speakers, and German speakers have better knowledge of English than Polish speakers according to this map.
Also, I'd like to say that I think accent is irrelevant in a discussion about proficiency in English. I'm not sure if you wanted to imply it is, but that's how I read your post.
You're absolutely right in saying that there are more German speakers than native Polish speakers and that German speakers are more proficient in English than Polish speakers. But that to me seems like an exception to the general rule of thumb.
Accent is indeed irrelevant, because everyone has an accent. What I actually meant was that people from the Netherlands, including academics, mispronounce words. One of my professors pronounced "access" as "excess", and MIStake, rather than misTAKE. Quite an error if you ask me. I hear quite of a lot of that: placing the accent on the wrong syllable.
Here's another tidbit. At my university, American textbooks (and texts from the UK to a lesser extent) are the main choice for science courses. In Spain however, the very same textbooks that we use are translated into Spanish. I guess that makes sense: there are more Spanish than Dutch speakers, so the market is much larger. But as a consquence of that, Spanish speakers don't even develop passive English skills, nevermind being able to communicate fluently. I guess they could learn English equally well, but since pretty much everything is translated into their native language, they don't have to make any effort to learn English.
The relationship runs both ways. It makes little sense to translate textbooks into Dutch because:
1. the Dutch have excellent passive skills.
2. the market is very small.
Having a small market, proved to be an advantage for the learning of English. The same applies to Danish, Swedish and so on. I doubt there are advanced Cell Biology books in Danish.
I just don't think it's that simple.
One factor might be the education system- there are many Finnish people who are at native speaker level and that may be down to the fact that they start learning very early, along with the professionalism of their teachers and system.
In the case of Poland, the Czech Republic and other countries in that area, I was teaching in the UK many years ago when the first students in the post-Soviet era came over. They were noticeably behind their EU counterparts- their teachers had, in some cases, been teaching Russian as the second language and were suddenly dusting off English books to teach that a few lessons ahead of their students. However, the gap closed astonishingly quickly- within four or five years, there was no difference. To pull this off, I imagine there were a number of factors at work.
Tdol is right. The fact that communism regime plagued countries such as The Czech and Slovak Republic, Poland, Hungary etc. make the learners of English in such countries somewhat fall behind those who have lived in a country never plagued by communism. You see, for the communists, English was, as they used to put it, "an imperialistic language of the evil West". Therefore there was no point for the inhabitants of such countries to learn it, because they would never need it. Russian was the main second language an sometimes German. Sadly, the impact of communism on the proficiency of English learners (and the penetration of English as a second language) in the post-Soviet countries still (more or less) prevails.
Note then, that this is only of of the factors playing part in this issue.