On my Wordpress blog, I want to show additional content to people from Finland on all pages. Very much like the Feedback button at the left edge of the screen on printfriend
My suggestion would be to use the PHP GeoIP functions to determine the users location based on IP, and serve appropriate content based on that.
More information on GeoIP can be found here.
In addition to the other answers (GeoIP), you may try to analyze the Accept-Language
header field and serve the content to users that accept finnish language.
You can modify the following function to parse Accept-Language
header in PHP:
As yet another alternative, you can download one of the SQL databases here and use it to resolve an IP address to a location:
http://www.ipinfodb.com/ip_database.php
Also, they offer an API that does the same thing, but this may not be optimal if you're getting a lot of traffic.
Here is a pointer to MaxMind GeoIP Country Database. http://www.maxmind.com/app/country. It's about $50.00 and comes with a PHP library for accessing the information.
You have a few bets you can try:
Hope this helps!