In regards to swearing, I'd say that there's nothing wrong having something like that in a fan-game, but I would definitely keep to a minimum. If everyone's swearing like sailors, it can be a bit of a turn-off to some players, and they'd stop playing. Likewise, if they're an evil team leader and their plans for world domination have just failed, I don't think that "oh gosh darn" really sells it.
In short, when it comes to swearing, just don't over do it, and I think you'll be okay.
As for 18+ content, eh... might be a bit overboard, IMO. Personally, I'd try to avoid having that type of stuff altogether. It'd feel pretty out of place, especially in a Pokemon game. Not to mention that a lot of people probably shouldn't see stuff like that, especially considering a large portion of the Pokemon fan-base is under 18.
Ultimately, it is your game, so you can do what you wish with it. Though it's definitely beneficial to consider what your potential players want to see and not see. It might be best to lean away from stuff like that as much as possible.