Most medievalists I know use the AutoCorrect feature of Word (yeah, I know) as a sort of completion feature. It’s pretty handy to type “xlatn” and get “translation,” “CdT” and get “Cr?™trien de Troyes,” etc.

Back in OS 9, I had a freeware program called Text-something that did this is all applications, but I’m not aware of an OSX equivalent.