Ask HN: How do you test locale specific web apps?

I recently ran into an issue where an ad was displaying fine locally (in the US), but there were reports of no ads being displayed in the UK. I used a free UK web proxy to see the issue for myself, but there were no client errors that I could see so I wasn't sure the best way to debug the issue.

What do you use to test/debug your apps that are dependent upon the locale/country it is used?