Complete text of Durga Saptashati is available in Sanskrit language.
You can read Durga Saptashati both offline and online mode.

Durga Saptshati Path also known as Devi Mahatmya or Chandi Path is a famous Hindu scripture describing the glory of Goddess Durga. Goddess Durga is considered as the supreme power and creator of the Universe. Full Durga Saptshati Path contains 700 versus divided into 13 chapters or 13 Adhyays. Durga Saptshati is part of the Markendeya Purana, a holy Hindu scripture written in Sanskrit around the 5th century. The powerful Durga Saptshati Path also contains Devi Bhagavata Purana and Shakti Upanishads such as Devi Upanishad.