Trisquel was born in the Galicia region of Spain, so the primary language was Galician, and Castilian and English were also included. In version 2.0, Catalan and Basque were added, and in version 3.0 Portuguese, French, Hindi, and Chinese were added. From version 4.0 forward, only English and Spanish are installed by default to save disk space. The default language is English, but you can select another from the outset.
The installer lets you select numerous other languages to be downloaded and configured during the installation process. Extra languages can also be installed from the Main Menu (bottom left) via System → Administration → Language Support.