Reincarnated as a Slime -- Are the alphabets in the end credits real?
Looks like an original alphabet, possibly based on runic alphabet. However, it is a complete alphabet with known transliteration.

解読マリィ@happymaryheart tweeted his work on the complete transliteration of the alphabet.



Based on transliteration samples he tweeted, it's basically either:


English words, or
Romanization of Japanese words




As for why the credits are shown in three alphabets... it's actually only shown in two alphabets: this runic alphabet and Japanese/English. The runes are shown for the first time, then they shrink before being replaced by the original words.

As for why it's done this way... I haven't done any researches for this, but I'd speculate on artistic impression, or perhaps it's actually the only alphabet in their universe...