The quickest way to change your language settings is to perform a search at Google.com first. You can find it in the upper-right corner of the search results page. This will open the Google Search Preferences page. If the language is set to one you don't understand, the "Save" button is blue. This will change the language for all Google websites, including YouTube and Gmail. If you aren't logged in with your Google account, the settings will stay until you close your browser session. If you are signed in with your Google account, the language changes will saved and loaded every time you log in. By default, changing Google's language will change the search results language as well. Click the "Edit" link to select all of the languages that you want to see your search results in. You can select more than one language.

Summary:
Perform a search at Google.com. Click the Gear button on the search results page. Select the "Languages" option. Select the language you want to use and click "Save".