What is the ruling on saying "yarḥamaj Allah" to someone who sneezes, given that some people change the letter kaf to jim?

Responding to a sneeze does not require the Arabic language, as it is a supplication (du'a), and supplications are permissible in languages other than Arabic. Allah knows the intention of the supplicant. If it is permissible in languages other than Arabic, then its permissibility in one of the Arabic dialects is even more so. However, the most preferable is to respond with the prophetic wording: "Yarhamukallah" (May Allah have mercy on you).