https://github.com/alexpovel/srgn It grew out of a niche, almost historical need: using a QWERTY keyboard, but needing access to German Umlauts (ä, ö, ü, as well as ß). Switching keyboard layouts is possible but exhausting (it's much more pleasant sticking to one); using modifier keys is similarly tedious, and custom setups break and aren't portable. So this tool can do: $ echo 'Gruess Gott, Poeten und Abenteuergruet…
Did you opt to blacklist words with vowel combinations that should not be transformed, or whitelist those that should? Or something else entirely?