Should be done in a way so that developers can implement their own transformation. You would then provide just the most common ones. Some language scripts have special algorithms in regards to casing.

And should not stop at casing either. Maybe someone wants to convert the numbers in text between various numeral systems. Or romanize a non-Latin script. Or fiddle with text attributes. Not sure if it's viable, but the way I see it you would just provide access to every character in text, with characters' attributes (font, size, style, even is_selected flag), and with the ability to change any of it. And after the changes you "just" merge characters with identical attributes to layout boxes and reflow the text.

Hi, could you implement it like Microsoft have done https://support.office.com/en-us/article/Change-the-capitalization-of-text-9e3a6b90-5aa4-4fe9-85da-2852605c57af

Thank you