I type a lot in other languages, but sometimes it's just a word or two and it's a hassle to go through and change the whole keyboard settings and back again... is there a way to long-press a key and have international characters pop up?
I do not know if something like what you search for is implemented (maybe you can explore something about "input methods"), but the traditional "Unix/x11" way is to define and use a "compose key".
The idea is you have a special key that starts a sequence that will produce a specific symbol you normally don't have on your keyboard.
These instructions refers to Gnome Desktop, but I suppose it is the same in all the desktop flavors. To activate a compose key, go to settings -> Region and Language and then choose the "layout" tab. Click on "options" and choose where you want your compose key to be:
Now you can enter strange chars like that: for example, press "compose", then "a", then for example "~" and you have ã (the font you use can have the glyph or not...).
A full list of the characters you can obtain via compose can be found, for example, here: