With the decision to start writing the client first came the question of client technology. The existing Eressea client, Magellan, is written in Java, and even if I wanted to use some code from it, I would not be able to run it on a mobile device. I also do not know the code base, since it is mostly written by player volunteers.
So unlike the server situation, there is no reason to stick to the language used in the past, and I am free to choose whatever I want. The real constraint here is mobile, and the options that I know of are Cordova/Phonegap (HTML5 and Javascript), Unity and LÖVE.
Cordova is the only one of these that I have practical experience with, and while it’s a great choice for an app with a lot of UI, it is not what I would choose for a game in which I want to render a lot of bitmaps. I am also not a huge fan of Javascript. Unity is hot with other indies, but it is a 3D engine first, and there is a learning curve that I do not feel like scaling right now. I would also have to brush up on my rusty 3D math, just to make 2D projections. That seems like a waste. On the other hand, Unity skills are very marketable in the game industry right now.
Finally, there is LÖVE. I had only briefly looked at it before, but it looks exactly like the engine that I had always wanted to write myself: Built on solid, industry-proven, portable C libraries (SDL, PhysicsFS, Box2D), with Lua as the programming language, and Android and iOS ports in addition to the desktop releases. There is an active developer community, and an IRC channel where people answer questions. I think it’s charming, so I wrote a few prototypes for key technology (networking, UI, controller input), got a simple map viewer for Tiled to run on my phone, and decided that my search for technology was over.