This report lets you see which languages your visitors speak. For web visits, Language is derived from the visitor's browser locale.
Language along with location can provide some valuable information. For example, if you discover that the majority of your visitors are from North America but also speak Spanish, you might want to create a separate Spanish-language version of your site and focus more of your ad spending in Spanish markets. A similar principle would hold true if you discovered that visitors who spoke Japanese converted at a higher rate than visitors who spoke English.
You can use Segments to compare traffic such as Visits from iPhones and Visits with Transactions to see whether there is a correlation between language, mobile access to your site, and transactions. If there happened to be an increasing number of visits from mobile devices, and those visitors showed a tendency to complete transactions, you could create a separate mobile version of your site.