1. I can tell you, as a student of linguistics, that languages differ not only in what words they use to describe things, but also how they sound. In other words, all languages have rhythm. To the western ear, japanese and chinese might sound like the same gobbledygook, but if one listened closely, one would perceive the musicality of these two languages, so much so that, even if you couldn't speak either of the two, you would be able to distinguish one from the other.

Am I suggesting that it's not we who change personality between languages, but that it's the languages themselves who have personalities of their own? Kinda. Historically, we tended to associate certain languages with certain human traits: german is the language of hate, french is the language of love, etc.

2. Native speakers have certain anchor expressions (not a technical term) in their speech. Terms that link parts of the sentece together. Like uhm or like, for english.

"And, uhm, we just went there." "She was, like, completely disgusting."

Generally, english-as-a-second-language speakers tend not to use those. It's not something that can be taught. You learn it by hearing other people use it. So, perhaps, I'm speculating, your confidence and security may come from the absence of these little words. They increase the fluidity (again, not a technical term) of speech, so their absence may increase the seriousness and gravity of it.