If anyone knows a touch based way to add diacritics to text I would be interested to hear about it..... WritePad comes close but I don't see any accents or diacritics.
If I interacted with twitter via write pad I might actually use twitter. I have hated every interface I have seen for twitter to date. My opinion about the interface has resulted in my reactions to the the object. In the case of Twitter, this has largely resulted in me having a twitter account but never using it. Interface and interaction design has kept me from engaging in a particular manner. I wonder if there are things in your world which would change if you interacted differently with them.
BTW: This second one is awesome too. Completely different but requires a keyboard layout. One reason that keyboard layout research is still important.
Edit: Nov. 19th 2014
Comment from Kari: If you keep your finger pressed on a key, e.g. [e], the diacritic possibilities pop out: [é] and another one. Then you just choose the right diacritic.
Response to Kari: Right, that is on iOS, and it works great for the supported combined Unicode characters. Not so great for with characters which require combining diacritics. (See the video here for an example: http://www.macworld.com/article/2036310/type-special-characters-in-os-x.html) When I wrote that bit above I was thinking about diacritics being input by hand script like is done in write pad. As it exists it would require adding diacritics after the handwriting is converted to text.
The following video shows the phonetic input method for Chinese Characters. The challenge with this method is that not all pinyin pronunciations match the way people speak around the country.