For those who wish to help with translations and don't want to install the quite large Qt SDK (a few hundred MB), there is a 6.4MB Download of the necessary tools available here
There is one caveat, though: Since binaries for V0.50 haven't been released, you cannot check your translations inside Sigil yet - above tools don't allow you to create your own build.
As a sidenote: On Windows, you'll have to use "lupdate ../../ -ts out.ts" to produce the TS file. Note the missing asterisk. I believe this command line should also work on Linux, but haven't tried.
BTW, I think it makes sense to include the sigil_en.ts file in the GIT repository so that translators don't have to pull the whole source and create the TS file individually. This could also be part of a "string freeze" process.
P.S. I'm working on a german translation. If somebody else is doing the same, please contact me via PM.