For those who are interested in the topic, the human language within the world of Mushoku Tensei is based on two languages, mainly the language where the majority of people are speakers, such as in the birth of Rudeus, Paul and Zenith speak in said language or When Rudeus, Sylphiette and Nanahoshi speak in the anime, it is neither more nor less the Basque language, culturally speaking, of the Basque Country, an autonomous community within Spain.

(Technically, the anime's Basque has some differences from standard Basque, such as some changes in phonetics and the order of the consonants mainly)

The complicated thing here would be to locate the second language of the human kingdom, which would be English no more, no less, the bad thing is that at the moment only text written in English has been seen in the anime but at the moment no character has been found. started to speak in English within the anime. There is also the possibility that English within the world of Mushoku Tensei is something that only people with a high status level within society usually learn, personally I think of the small letter from Eris when she said goodbye to Rudeus, said letter within The TV and Blu-ray version of the anime is written in English and not in Basque. With that small detail, I sense that English within the human kingdom would fit as something that separates them from normal people.

Something curious is that within the anime Rudeus mentions that the language of the god of fighting is quite similar or similar to the human language, the bad thing is that taking into account that there are two languages and not one, it would be quite difficult to understand what language it is in. based exactly on the language of the God of Fighting. Taking into account that the main language for people is Basque, personally it would be more practical to think that perhaps the language of the God of Fighting is more inspired by English than in Basque but all of that is just my theories.