As with other programming languages, it is not very straight forward. A commonplace solution for UTF8 is to use a long-stroke overlay. However, that works character-by-character rather than word-by-word (or whole sentence) and the results are inconsistent (= can be ugly). So you would need to experiment to see if it is suitable for your purposes.
Another approach would be to work with HTML and CSS. App Inventor can display an HTML page (on the device, no internet required). The CSS methods are described here: