I did Oseka's Knowledge in Rubak where you inspect murals and each time you touch one, it'll increase your Hadal language by 25. I had only did 2 so that my skill was 50. On my character page, Skills & Effects :: Lang :: an unknown tongue was also 50.
I recently had a chance to finally touch all 4 murals and my Hadal Language is now 100 and "an unknown tongue" is also 100.
