The French accent sounds nasal because you use nasalized vowels, such as in the preposition "dans." Certain American accents are known for nasalizing vowels somewhat, such as the northern US (Michigan, Chicago) where a word like "man" starts to sound like "myan" when it is pronounced.
True the French overall is definitely nasal. While there are certain American dialetcs, but obviously not all, that do sound a bit nasally.
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Why "weird "?
You don't find that French language has a lot of nasal sounds ?
Such as : /-an/, /-in/, /-ont/,.....etc.
I love the American accent when it's nasal. It's so cute. I love to listen to my boyfriend just for his nasal accent. It's so charming.
Some American accents are nasal but different kind of nasal than French language, for instance.
Well what is weird is that the Americans often don't think their accent is nasal (but they will tell you that the accents of some American regions different of the one in which they live sound nasal).
But the same thing goes in France: many people who use the Parisian accent will say that only the southerner French accent is nasal, and no French southerner find their accent nasal.
I guess the kind of nasalities we (French and American) use are very different, and that why we remark those we don't use as though their were predominant.
American accents seem nasal beacuse they are formed further back in the mouth then british accents, try holding your nose and talking in an american accent, its quite hard but talking in nan english accent is easy.