I commented on the forum recently (Technical and genealogical contacts #post 4595) that I was looking at a way of simplifying the creation of “custom language files”.
Well, I have succeeded, at least to an extent, and this will be a new addition in the upcoming release kiwitrees 3.1.
Here is a picture of the edit screen:

A full FAQ for it is here: http://kiwitrees.net/?p=4642
A couple of key points:
- It is an alternative not a replacement for the old file-based custom language files. Any of those you have will still work.
- Like those (except ‘.mo’ gettext files) it can only be used for simple texts, not those with gender based alternatives, plurals, data variables etc.
- This stores the translations in the database and requires no special files at all. No need to FTP anything to / from your server, nor worry about what text editor you use or getting the code right.