Answer below the dotted line below is the original that's now outdated.
Here is the latest information ( Thank you @deadfish ):
add &hl=<language> like &hl=pl or &hl=en
example:
https://play.google.com/store/apps/details?id=com.example.xxx&hl=en or https://play.google.com/store/apps/details?id=com.example.xxx&hl=pl
All available languages and abbreviations can be looked up here:
https://support.google.com/googleplay/android-developer/table/4419860?hl=en
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To change the actual local market:
Basically the market is determined automatically based on your IP. You can change some local country settings from your Gmail account settings but still IP of the country you're browsing from is more important. To go around it you'd have to Proxy-cheat. Check out some ways/sites:
http://www.affilorama.com/forum/market-research/how-to-change-country-search-settings-in-google-t4160.html
To do it from an Android phone you'd need to find an app. I don't have my Droid anymore but give this a try:
http://forum.xda-developers.com/showthread.php?t=694720
