Lashon Hakodesh means the 'Holy Language' or the 'Holy Tongue' and refers to Biblical Hebrew that was uncorrupted by the Reform movement. It also refers to all types of 'Holy Speech' such as Torah, prayer and Hisbodedute, even if they are spoken in a foreign language.

Holy Speech Lashon Hakodesh - also means holy speech such as Torah, prayers and Hisbodedute, even if spoken in a foreign language. Holy speech cools down impure passions and one merits Tikkun Habris, thereby subjugating the Primary Lust of immorality.[1] Since holy speech and Tikkun Habris are dependent on each other, the reverse also applies and a person who reaches Tikkun Habris will merit to perfect his holy speech. When a person reaches higher levels of Tikkun Habris and Lashon Hakodesh then his face radiates. When others look at him, it works like a mirror and they realize how they are in the steeped in darkness. This causes them to repent on their own without even the need for any rebuke.[1]

[1] Hishtapchus Hanefesh 70