When determining where to show your ads, the AdWords system looks at a user's Google interface language setting to see if it matches one of the languages that your campaign targets. For example, only users whose Google interface language is Spanish will see ads in a campaign targeted to Spanish. On the Google Network, the AdWords system looks at the language of the webpage and shows ads targeting that language.
About Google interface language settings
Each Google domain's interface language setting defaults to the language most commonly associated with that Google domain. For example, Google.com defaults to English, Google.fr defaults to French, etc. Users can change their Google interface language via the 'Preferences' link next to the Search Bar on the Google homepage. A Spanish-speaker living in the United States, for instance, may want to perform searches on Google.com but change the interface language setting to Spanish.
Note: Some users may also choose a specific search language on the 'Preferences' page. For example, a user who only wanted Google to return Russian page results could check 'Russian' in the 'Search Language' section. In this instance, your ad would only show to this user if your campaign targeted Russian.
Targeting multiple languages
You may consider targeting multiple languages to expand your advertising reach on Google. For example, an advertiser may have a website written in English and an AdWords campaign with English ads and keywords. However, he may choose to target that campaign to both English and Spanish. This way, his ads will show to bilingual users who search on his English keywords -- indicating that they speak English and would be interested in his website -- but whose Google interface language is set to Spanish.
Targeting multiple languages can also increase your ads' exposure across the Google Network. A website in the Google Network can target a certain language, meaning that only ads targeting the same language can appear on that site. For example, a content site in the Google Network may target the Spanish language, which means that your ad would only show on that site if your campaign's language targeting selection included Spanish.
