Can no longer get alternate (international) letters by holding down a key

I use special characters quite a lot, since I work with museums and will be using spellings in several different languages. Adding special characters with accents, umlauts and the like used to be easy - I'd just hold down the corresponding key on the keyboard and the alternate characters would pop up, I'd choose one and everything was hunky-dory. No longer. This is a brand-new Mac mini, but it was working when it was first set up two weeks ago, but seems to have ended with a recent update to MacOS. I see how I'm supposed to check in the keyboard settings, but there's no specific setting to enable this function. This is really causing a lot of issues for me ... was this function removed for MacOS users? I can't imagine that could be true.