The problem with fictional languages is that no one actually understands them. This causes two issues that must be solved: first, any audience will have no way of knowing what any of these fictional words mean, and second, different peoples that speak different fictional languages need some way of understanding one another.
While it may make sense for a fantasy world to use some magical tongue, or an alien world to speak an alien language, audiences simply will not understand any narrative that uses that language. There are several solutions to this problem. The first is to ignore it and have characters speak a language that can actually be understood. The second solution is to make the audience aware that a translation is occurring or that the characters of this world are not actually speaking the language that the audience is hearing. This can be done in several ways, such as beginning in the fictional language and transitioning to one the audience understands. Or using an understood language but showcasing minor language elements such as greetings or text written in fictional alphabets so audiences recognize that while they are hearing a language they understand, it may not be the language ‘actually’ being spoken. Subtitles and an audience that enjoys really, really foreign films are another potential solution.
Worldbuilders with more than one fictional language have another problem they must face: how can people that speak these different languages understand each other? Solutions to this dilemma include inserting a translator or incorporating a common tongue.
Most worldbuilders opt to ignore this problem as audiences have largely internalized the need to use English or other real languages in fictional settings that would not realistically speak those languages.