This depends heavily on the language (and regex engine) you're using.
In Perl, \w matches all word characters, regardless of language or alphabet, and something like /\b(\w+)\b/ would (probably) match Spanish words as well as English words or Russian words.
In languages using PCRE, \w (and therefore probably \b) do NOT match Unicode characters. You will probably need to build your own set. I suggest something like [\wáéíóúñ] (matches all word characters, plus the accented characters you want), and the PCRE library has to be pre-built with Unicode support before this will even work.
If you're using something else, good luck. Some regex engines don't even support Unicode.