Your ad can show to a user when your targeted language matches:
a user’s browser setting
a user’s Google interface language setting
the language of websites a user visits most often
a user’s operating system language
Each Google domain has a default language. For example, Google.com defaults to English, Google.fr defaults to French, Google.cn defaults to Chinese and so on. The default language can be changed via the Settings link on the Google homepage. A Spanish-speaker living in the United States, for instance, may want to perform searches on the US domain, Google.com, but could change the interface language setting to Spanish. In that case, they would see ads targeting Spanish instead of English. While your campaign always targets searches with a matching interface language, your campaign may also target searches or Google Display Network sites that we detect are written in your target language.
Your ads can appear for customers who use Google products and third-party websites in the languages that your campaign targets. This helps ensure that your ads will appear on sites that are written in the language of the customers you’d like to reach.
Let’s say you sell coffee beans online, and you want to target Spanish-speaking customers. You set up an AdWords campaign targeted to the Spanish language, with Spanish ads and keywords. As long as your customers’ Google interface language settings are set to Spanish, your coffee ads can show when your Spanish language customers search for your keywords. Keep in mind that if your customers searched in Spanish but their Google interface language settings were set to English, your ads wouldn’t show. That’s why targeting all languages might be helpful.
Read more here: https://support.google.com/adwords/answer/1722078?hl=en