Lisandro,
in case you are still having these issues – or anyone else ends up on this page – the following.
Like Jan pointed out these issues have to do with the charsets.
In most cases the charset definition in wp-config.php must be UTF8, just like the variable for charset in wp admin read settings. Collation will normally not be set in wp-config.php, which leads to the default utf8_general_ci to be used.
Note that charset and collation are also set on several levels in your database.
As for plugins and themes: most important thing to check is whether a meta tag with charset definitions is added to the head of your pages.
In your cases you can consider to change the collation to utf8_unicode_ci for its better sorting performance. Best to have someone with working knowledge to do this.
Fairly good information on this can be found at http://codex.wordpress.org/Converting_Database_Character_Sets
Cheers,
Peter