To boost results for your region, click the region button - - above the search results.
You can also change your region setting on the settings page.
On a per-search basis, you can change your region setting by adding r:code to your search. (For example, r:fr for France.) The full list of region codes are on the parameters page.
Changing your region setting will boost the ranking of search results from that region, as opposed to exclusively displaying results from that region. You can restrict your results to a country's domains by adding site:ccTLD to your search. A ccTLD is the ending of that country's domains, such as .uk for the United Kingdom. So for example, adding site:uk to your search would only show results from .uk websites.
Unfortunately, we don't have complete control over the options available for the region setting. We rely on some external sources to help us with them - but we're constantly monitoring our sources to allow for more regions!
Prior versions on GitHub.