This report lets you see which languages your users speak. For web data, Language is derived from the browser locale.
Language along with location can provide some valuable information. For example, if you discover that the majority of your users 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 users who spoke Japanese converted at a higher rate than users who spoke English.
You can use Segments to compare traffic such as Mobile Traffic and Sessions 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 in traffic from from mobile devices, and those users showed a tendency to complete transactions, you could create a separate mobile version of your site.