A good article about charsets can be found at http://codex.wordpress.org/Converting_Database_Character_Sets.
Unless you have very specific requirements to the languages you are using, you will want to use charset utf8 with collation utf8_general_ci.
The default WP settings are OK for this. I.e. charset definition in wp-config.php will be UTF-8. No collation specified in that same file.
In WP admin read settings, encoding will be set to utf-8 as well.
Check your database and database tables; these need to be set to have collation utf8_general_ci. If you're working with hosted WP, there's a fair chance these will have latin1_swedish.
If you dont have direct access to phpmyadmin use a plugin like adminer to get access thru wp admin.
If you need to convert any of your tables, check the article mentioned above.
If your database isnt too big, you can consider just converting and retyping the special characters.
This should solve your problems. If not, let us know!