Héctor,
The truncate function uses substr (in 3 places) to truncate the text for display. substr is not multibyte character compatible. If you happen to have an accented or other multibyte character that should be the last character of the truncated string, it gets cut apart by substr and shows as a weird character. Replacing substr with mb_substr (which is multibyte aware) fixes the issue.
http://wordpress.org/extend/plugins/wordpress-popular-posts/