The Writing System in Tensura Universe

INTRODUCTION

For a fantasy story that tells about another world (or isekai), the presence of artificial language(s) and/or writing system(s) will make the readers feel as if they really go to a world that is completely different from the real world.

A series called "Tensei Shitara Slime Datta Ken" (shortly "Tensura") or "That Time I Got Reincarnated as a Slime" is no different. If you pay enough attention to the everything shown in some scenes in the anime or manga, you'll probably find some weird writings there.

What kind of writing is that? What language is it?

The anime implied that the language used by three adventurers (Eren, Kaval, and Gido) was English, as Rimuru said in episode 2, "I was never good at English."

The LN, though, hinted that there wasn't only one language. In the first volume, the language used by those adventurers and the goblin language were mentioned as different matters. The prologue of the ninth volume also stated that the language used by people in a town where Hero Masayuki accidentally transported to (from Earth) was complete gibberish to him. If it was English, technically speaking, people who came from Earth should've known immediately.

Considering that Eren (from Sarion), Kaval, and Gido (possibly from Blumund) were from different countries and they could speak to each other with English, it could be assumed that there were really various languages with English as the lingua franca (just like the real world, really). This matter was never explained further in the LN.

Since it was another world that was supposedly different from the real world, it wouldn't make sense if the people there also called the language as "English". Because it was used as the lingua franca, we could just call it the World Language or 世界語 (Sekai-go).

Long story short, World Language in Tensura universe is basically English, but using a different writing system. How can we read it?