What language did God write the Ten Commandments in?
According to the original scrolls of the Old Testament the ten commandments were written in Hebrew. Presumably as they were given to the Jews, and Hebrew was their language and however presented to them needed to be understood, I don't think there is ever a question about that.
However, there has been, and remains, a heated academic dispute about if the scrolls/tablets were written by G-d and handed to Moses, or told to Moses and written (more properly I guess etched), by him on the mountain.
Current Archaeological View
If current archaeological beliefs are correct, there was no Exodus from Egypt. Therefore there was no event of receiving the tablets in the 12th century BCE. The tablets (if they existed) would have been hand-carved centuries later as the Israelite tribes were establishing themselves in the Judean highlands, but at some time before the Davidic Kingship because the Jahwist source lists the commandments in Exodus ch. 20. If this were the case, the tablets would have been written in the Paleo-Hebrew alphabet (which has been dated to the 10th century BCE and possibly earlier). The Hebrew language, though, would be roughly the same as what we find in the Old Testament, just with a different alphabet.
You cannot mix the Biblical story of the Ten Commandments' revelation with the archaeology to come up with a tablet written in the 12th century BCE in Hebrew and date it to the Israelite civilization. Either you accept the Biblical account or you go with the current archaeological theories.
The Ten Commandments contain universal moral codes that can be found in almost all religions. It has been said that they are largely a repetition of the Babylonian Code of Hammurabi and the Hindu Vedas, among others. Where they do differ is in commanding that the Jews have no other god before me [God].