I don’t know if “subiectum” is the equivalent of our modern “subject,” maybe someone could clarify this, but in case it is, I would render the phrase as follows: “Subiectum est de quo sententia loquitur.”
Could you explain how your sentence works, sis? The ablative de quo gives me trouble. Gratias tibi ago.
(According to John C. Traupman’s Conversation Latin and the Cassell’s Latin Dictionary, subiectum is the Latin word used for the English grammatical term.)