Press Fn+VO+F8, or Fn+VO+8 if you are unfortunate enough to have a TouchBar model, to open VoiceOver Utility. Then interact with Utility Categories and activate Speech. Stop interacting with Utility Categories, make sure you are in the Voices tab which is the first so should be selected by default, and there you will find buttons to add and remove languages.

Once you're done adding another language, you can select it by pressing VO+Command+Shift+Left/Right until you reach the Voice rotor and then use Up/Down to select the voice. You can also make it automatically select the voice based on the language of the content, but I prefer to have control over this because there are websites in Portuguese (my native language) that either have their Content-Language incorrectly set or have no Content-Language at all, which causes VoiceOver to default to en-US since that's the language that I use on MacOS.

You may also want to add another keyboard layout in Keyboard Preferences. I have a US ANSI keyboard which is much better for coding than the Portuguese ISO keyboards, but when I want to type text in Portuguese I have to switch layouts since accents aren't dead keys in the US layout and it's also missing the C with cedilla.