Elmesia's title is 天帝 or 皇帝 in Japanese, and both of these are gender-less words.
In any case, the English people probably had the same thought you did and defined Kingdom as being able to be headed by a queen also. The United Kingdom was one example until recently.
There is a reason Rimuru is special besides being from a different world, and is related to the mask, but that is the next story arc for the anime (LN vol 11) I think.
Why is there no 28 Months Later?