How do you convert bits into a different alphabet?
To estimate the number of decimal digits needed to represent a $128$ bit number you use logarithms to base $10$:
$128 \times \log_{10}(2) \approx 38.53$
so you need $39$ decimal digits to represent a $128$ bit number.
In a general, for an “alphabet” with $n$ symbols you need to find the value of $128 \times \log_n (2)$ and then round this up to the next whole number.