Complex sentence: In a document produced via single-sourcing, however, the index will be translated automatically and the two words will be rendered as synonyms because they are in the source language, while in the target language they are not.
Split into simpler sentences: In a document produced via single-sourcing, the index will be translated automatically and the two words will be rendered as synonyms. This is because they are synonyms in the source language, while in the target language they are not.