You're correct that the word ريال can be rendered either way in English. But since the Yemeni 20 Rial coins specifically say "20 RIALS" in English on the reverse (directly below the Arabic "ريال٢٠"), I think it's clear that's Yemen's preferred English translation.
Well - I often like to choose the native used spelling.
There are lot of exceptions though...
As for russian / east european coins - which would normally be in most cases something like
Rubel' or Rubley etc. - used here is the term Rouble.
so... we might change it to Rial instead with Rial (Riyal) in the "word form" field.
