Back in the Old Days™ ASCII was king and it was pretty much impossible to faithfully render any language that required diacritical marks or non-Latin characters. So essentially you could render English but anything else was iffy. All that changed with Unicode and especially UTF-8, which solves the compatibility issue by having ASCII and its representation as a subset.
These days, practically every application supports at least UTF-8 and Emacs is no exception. As you can see by typing Ctrl+h h, Emacs can faithfully render any language you’re apt to meet. That raises the question of how you enter those characters.
Mickey from Mastering Emacs has a nice post on how to do that with particular emphasis on dealing with diacritical marks. For me, the easiest way to deal with diacritical marks and the like is to use the TeX input method. It allows you to enter text pretty much as you would with TeX or LaTeX. But there are other input methods that essentially give you a keyboard for whatever language you need. For one-offs, I tend to use John Kitchen’s ivy-insert-org-entity that I stole from his Scimax code. It’s perfect for putting in a single diacritical or a special symbol such as the ™ above.