Complex sentence: Under King Alfonso X of Castile during the XIII century, the translators no longer worked in Latin as the final language, but a revised version of Castilian, establishing the foundations of the modern Spanish language.
Split into simpler sentences: Under King Alfonso X of Castile during the 13th century, the translators no longer worked with Latin as the final language, but translated into a revised version of Castilian. This resulted in establishing the foundations of the modern Spanish language.