Update descriptions are not internationalized

[mikehearn]

Descriptions for online updates are not translatable currently. This would require changes to UpdateFX and the general infrastructure.

It probably doesn't make much sense to hold bug fixes and other updates just to wait for translations of what the update changes. Web apps don't work that way, for example. So this needs careful thought to figure out the best way to do things. As the app re-downloads the update index each time it starts we can always set things up so translations can be added in retroactively, and a small library of vague/generic descriptions can be used to assemble a basic description if we have a way to categorise updates.