Tools like composer are here to make you life easier. As everybody already mentioned it automates installation of dependencies and also generates autoload file. That is everything that you need for starting your own project. You need code to solve a particular problem like library you mentioned, one command and POOF! you have it and you can use it. When you start from zero you can have all libraries you need for your project in a matter of minutes.
For this library to work without composer you will need to do:
- include manually library you mention in the post (Stichoza/google-translate-php)
- include all dependencies of that library (also dependency of dependencies and so on...)
- make sure you load all files (there are some options, you need to make list of all files you need from
all libraries and include them manually, or making some autoload class that will deal with that, or include in your composer.json library that is solving that issue, of wait you can not you don't want to use composer

), also some libraries are using multiple
psr autoloading standards (like library you want to use) so you will need to think about that also
- what if you do all of this and notice that you want to use some other major version on some library you have, your project will become nightmare
- and so on...
To make long story short, composer is a tool and should be used as such. It will save you a lot of time and will make your project easier to maintain and developer friendly. Composer is not perfect but it gets the job done. I use it on all of my projects no matter how small they are
