The languages of Isekai are varied, nearly every language has roots with a species or race that championed the creation of the language, and in many cases but not all, they are usually responsible for the creation of the script that the language uses. Many of these are available to learn for anyone that has the drive and desire to with the only limitation being the time and energy that you have available to learn. This is true for all but a few languages that have specific limitations, racial or class requirements that will require a greater investment than mere study in order to master them.
Most languages have two components, a written component, and a vocal component. Each of these aspects must be mastered in order to be fluent in a language and employ its full range of communication and benefits but there are many people throughout the world that just pick up parts of the writing or parts of the speech. This is often a long process and all but the most studious of adventurers are only going to learn 2 or 3 throughout their adventures, those that commit their lives to learn 6 or more are truly spectacular people that have seen the world or locked themselves up in a tower with books to accomplish such a feat.
Below are the languages found within the world of Isekai, all of them except for Dark Speech have some concrete way in which they can be learned. This method might be through books, native teachers, class features, or even explained in your background. Languages are a large component of the cultures throughout the many countries and cities in Isekai, some may grant you access to inner circles while others may provoke some to outright anger. Those that wish to flaunt their knowledge of a particular language would do well to understand its cultural implications, origin, and how it actually sounds to others before speaking it in a varied company.
Below is a list of languages that are found throughout the world and a short description as to who speaks the language and how it fits into the larger world of Isekai.