You can change the language and Country SERPs in two ways:
1. Within the Research, Compete, and Optimize applications (just for that application)
Click the flag icon in the query field in the Research, Optimize, or Compete applications. Select the country SERP against which you wish to optimize. The language will change to match the country SERP.
2. In the settings, where it will persist in all those applications
Click on the gear icon in the upper right corner while in any of the applications and make the change there.
The country SERP and language selectors are designed so that you can only select valid combinations. For example, if the selectors are currently set to United States SERP and English language, changing the country SERP to Bolivia automatically changes the language to Spanish.
Countries with Spanish SERP data are: Spain, Mexico, Columbia, Argentina, Peru, Venezuela, Chile, Guatemala, Ecuador, Bolivia, Paraguay, Nicaragua, and Costa Rica.