What are the reasons to use multiple programming languages in one software project? [closed]

As mentioned above I'm getting deeper into the actual application development and when I'm looking for technology stacks of companies I'm always wondering why they are using multiple programming languages. Wouldn't it be mostly reliable to use just one programming language to maintain a high degree of maintainability and the skill to refactor code form every developer? I do not really understand why it is so essential to use, e.g. 1) Python and Ruby in a Software Project or 2) Java and Go .

Can somebody give some insides why and depending on which factors you actually choose a Tech-Stack for a certain application.

Thanks for your help!