In terms of personality, I can actually agree with you about Tidus in the English dub. He sounds like a self-centered prick who doesn't really care about anyone but himself. At least, that's the vybe I get off of him due to his attitude and the way he talks to other characters. The Japanese dub Tidus, however, has a lot more emotion and actually cares about the other characters. You can hear it in his voice. Also, the scenes with Yuna, like where he laughs like an idiot, are a lot less awkward. He basically seems like a hero, a team leader, more than a selfish american who just wants his paycheck.
I don't really think the language matters, because at his core, which is shown rather blatantly at the beginning, Tidus is loud and annoying. This is intentional, as My Story is supposed to be a journey to manhood and finding oneself. Tidus, due to his relationship with his dad and the state of his childhood as a result, never really grew up. As such, he boasts quite often given that he's reached status of some sort and it gives him a false sense of having gotten a one-up on his father.
Even disregarding the laughing scene which, personally, I don't mind because the scene is meant to be awkward and silly- it's a way of showing that you can face adversity without fearing judgment, the whole point of the scene, it falls into the trap of him never growing out of that. A good number of games, anime, shows, and the like do this (a shining example is Mirai Nikki's Yukiteru Amano, a character that easily places lowest of anyone for me- though Tidus isn't nearly on his level), and the biggest problem is that...well, it isn't My Story, it's Yuna's.
And that's one of those things that I may just have to give the game credit for if it was intentional. All throughout the game, Tidus says that it's his story, and I think that that is a perfect reflection of his character. He really is childish to the very end, but it seems as if the game is very aware of this and that the whole idea of a supporting character forcing his way to the forefront of the story is simply a means to illustrate that. "My Story", as both a phrase and as a narrative, may very well be this tight clasp that Tidus has onto his childhood and, similarly, onto his father, that he refuses to let go of even after coming face to face with him and receiving...less than closure. By repeating that phrase, he is reminding the player that Tidus was as Tidus is, and Tidus will remain to be until Tidus can be no more. If that was intentional, then it's absolute thematic brilliance.
Regardless, however, my feelings on Tidus remain the same, and I don't particularly care for the way he's portrayed (Mainly because annoying =/= unlikable, and you can make a character that's both loud and childish and enjoyable to follow). If Tidus had been a side character and Yuna were the main, I'd probably have been able to tolerate him more. As a character in general, however, I mind him less thanks to the well written storyline between himself and Jecht in Dissidia/Duodecim (he's also toned down, probably due to the change in circumstances). In that game, I quite enjoy him and his portrayal, and it does a good job of illustrating the rather complicated relationship of the father and son.