A primary source is a document or other source of information that was created at the time of the events being studied. Secondary sources are documents or other sources of information that were created after the events being studied, and are based on primary sources or other secondary sources.
A translation of a work is a secondary source because it is not the original work, but rather a new version of the work in a different language. The translator may have interpreted or adapted the original work in some way, and may have added their own commentary or notes. This makes the translation a secondary source, rather than a primary source.