All science subjects are fine... especially math and physics...
xxx
I would replace the "biology" suggested subject with "geography". Biology includes zoology or botanics - dont need to know much about having dogs in a kennel with the bagage nor do you grow roses on the flight deck to please you lady captain...?
xxx
If you research my infamous background, you shall see that I have full fluency in a few languages, I would suggest that you study some French if your airline operates into Africa, or study Spanish for Central and South America. Maybe some people can massacre some English, within 1/4 DME of airports. Knowing the Cyrillic (Russian) alphabet, might be a help in Eastern Europe and Asia. I only know 50 or 100 words or sentences of Russian, but I can figure many words knowing their alphabet.
xxx
Of course, if you are a native Anglo-Saxon, stick to "English only" and ignore the other dialects spoken by some (or numerous) foreign ATCO... You see, we all know perfectly well that the language of aviation is English... at least that is what we are told during training...
xxx
Happy contrails