well actually ą is the same sound (or almost the same) as French on as in Bon voyage.
While the Polish "ą" is definitely closer to the French "on" than it is to the French "en", it is, however, not the same. This French nasal vowel is much "deeper" than the Polish one, since you start to nasalize the sound "o" at a much earlier stage, almost at the beginning of the sound, while for the Polish vowel the nasalisation phase is shorter and weeker.
Pronouncing "ą" as a sound similar to the French "en" can still be observed in Poland even if it is evidently wrong. It is often heard in the word "włączać/wyłączać" and such wrong pronounciation has recently been picked up by a TV advertissment promoting energy saving in Polish homes. In this ad, professor Bralczyk, a renown linguist, declares: it is not so important whether you pronounce wyłączać [en]
or wyłączać [on],
the most important is that you should turn the lights off [wyłączać światło]!
The writing of ą - as the a with a mark rather than writing it as the o with a mark has its historical reasons. At the time when Polish diacritical marks were being introduced to writing, this nasal sound was pronounced like the French "en", thus the letter "a" was taken as a basis for writing the sound down. Later the pronounciation changed to its present one [similar to the French "on"], but its written form remained unchanged.