Not in any reliable way. In theory a larger character set increases the search space for the attacker, but that only reliably helps you if you pick passwords at random (literally at random, for example by throwing dice), which I suspect is not what you have in mind.
But in practice, there's several problems. First of all, there's too many programmers out there who are incompetent and disdainful when it comes to character encodings, and routinely write software that doesn't handle them consistently. So you risk all sorts of issues:
- A website may do something funny to your passwords' diacritic-carrying characters—like replacing them with
? or just deleting them. This is terrible, because your password in that case doesn't have the strength that you expected and you wouldn't even be able to tell.
- Suppose then that such a website updates their software to fix the bug. Ooops, now it doesn't recognize your password anymore!
- Or the developers, without giving much thought to it, might decide to implement a front-end "permissible password" check that forbids non-ASCII characters.
Second, the way you're approaching the question can be boiled down to this:
- "Can I outsmart the password crackers if I use this clever trick?"
They're going to spend a lot more time studying how to outsmart users than you're going to spend studying how to outsmart them. They'll just read all they can find in the Internet about how people pick their passwords (including this question!) and learn from it to improve their password attacks. Remember, they don't just try to crack your password by guessing it individually—they program really powerful computers to test millions of passwords per second, and use them to guess thousands of users' passwords at a time.
So I don't know, but I sure wouldn't bet on you.
The only reliable way to defeat password crackers is to pick passwords at random from a very large set. The Electronic Frontier Foundation's page on creating strong passwords has good advice. The most important is to use a password manager program to generate and store a unique, random password for each site. That's it.