Character Analysis
No one can figure out what this guy is saying… and that's the way it's supposed to be. The actor who played Fenster—the always-awesome Benicio Del Toro—intentionally used an incomprehensible accent when delivering his lines.
This wasn't something the writer and director were planning on, but they decided to roll with it—even though it effectively eliminated the dialogue McQuarrie had written for Fenster. So thanks to Del Toro's creativity—and perhaps his inherent, brilliant weirdness—we all remember Fenster because of how incomprehensible he is. (Source)
Aside from this defining quirk, we know that Fenster's pals with McManus, and is one of the two crooks most responsible for leading them into the whole deciding-to-pull-off-the-emerald-heist-and-fence-their-stolen-goods-to-Redfoot mess.
But back to the accent…
Other characters demonstrate that they can't understand Fenster's speech, making it clear to the audience that we're not supposed to be getting it either. It's a running gag. For instance, one of the cops at the lineup says, "In English, please?" when Fenster recites the line they're giving him to read. Also, Hockney asks, "What did you say?" when Fenster mumbles something.
But why limit the character this way? Obviously it makes Fenster seem pretty one note. This is because Fenster plays no greater role in the movie than to die: Bryan Singer says,
I had to think, "Does anything this character say have any relevance or importance?" And then I realized, "No, his entire purpose is to die." (Source)
And die he does. After Kobayashi makes the gang an offer they can't refuse—work for Söze and pull off a drug heist or face the unpleasant consequences—Fenster tries to split. Söze has him killed, and Kobayashi sends the other guys to the seaside cave where Fenster's body is lying.
This demonstrates to the other characters that Söze ain't nothin' to mess with. And we take leave of Fenster as he lies in a sandy grave.