Vayigash
Yosef reveals himself to his brothers and to prove this says כי פי המדבר אליכם my mouth is speaking to you. What does Yosef mean to say? Rashi explains that “I am Yosef your brother since my mouth speaks to you with lashon kodesh.” Onkelos as well translates the words כי פי המדבר אליכם as “I am speaking your language with you.”
The Ramban asks: the lashon of Cana’an was lashon kodesh, so they all spoke it. How then is Yosef proving anything at all; maybe he was a Cana’ani? The Ramban answers that it is not a clear proof but rather an appeasement. The Nesina LaGer answers that Yosef showed the brothers that he knew their usage of words and the type of talk used uniquely by their family. This is hinted in Onkelos’ translation כלשנכון “as your language,” meaning how you speak its language, and not בלישנכון “with your language.”
In Parshas Vayechi Yosef requests to bury his father in Cana’an as he swore to Yaakov. Pharaoh responds “Ascend, and bury your father as you swore.” Rashi explains, if not for the oath, Pharaoh would not let Yosef go. Pharaoh was afraid to say transgress the oath and not allow Yosef to go, because then Yosef could respond the same about the oath which he swore to Pharaoh. Yosef knew all seventy languages plus lashon kodesh while Pharaoh only knew seventy languages, but not lashon kodesh. Pharaoh made Yosef swear that he wouldn’t reveal that he knows the language of lashon kodesh and Pharaoh doesn’t.
The Maharal asks, Pharaoh should have said, “Swear you won’t tell anyone that you are smarter than me.” Instead Pharaoh said “swear you won’t reveal that you know more languages than I do.” This shows he was more concerned with Yosef revealing that he knew one more language, than with Yosef being smarter than Pharaoh?
The Maharal explains that the function of a king is to be able to relate his nation. If he can’t talk their language there is something intrinsically blemished with his rulership. He is not fitting or able to rule over everyone. Yosef who knew all the languages including lashon kodesh was indeed more fitting for rulership than Pharaoh. Therefore Pharaoh made Yosef swear he will not reveal Pharaoh’s ignorance of lashon kodesh which reflects he was not as fitting to be king.
The Maharal asks, since Pharaoh was so smart and knew seventy languages why did he not learn the language of lashon kodesh as well? In addition, according to the Ramban everyone in Cana’an knew lashon kodesh, so why didn’t Pharaoh know lashon kodesh as well? The Maharal answers that lashon kodesh is a holy language and was given to Hashem’s nation who is holy and fitting. Pharaoh and the entire Cana’an were able to say the words of lashon kodesh, however could not understand its holiness and depth.
Based on the Maharal we can answer the original question. Yosef understood the language of lashon kodesh with its holiness and depth, as Yaaakov and the brothers, who were part of the chosen nation. Therefore when Yosef spoke the holy and deep language of lashon kodesh the brother recognized that it was Yosef!
May Hashem return us to Eretz Yisrael with the holiness and depth of lashon kodesh!