The Israelites will worship the Lord on this mountain after the exodus. A sign is meant to assure the people that God’s message is trustworthy. This is a long-term sign. In Exodus 4:7–8 God gives signs to Moses and the Israelites for the short term. The Israelites will worship the Lord. The word in Hebrew for worship is serve.
Now the Israelites serve (= do work) the Pharaoh in slavery, but they will be liberated to serve (= worship) the Lord.
12 Dieu dit: Je serai avec toi; et ceci sera pour toi le signe que c'est moi qui t'envoie: quand tu auras fait sortir d'Egypte le peuple, vous servirez Dieu sur cette montagne.