How to generate a single translation file for a large Qt project?

First of all split all your pro files into pro and pri files. Then put all the pri files into one global pro file and generate the language file for that pro file.

Special language pro file looks like this:

TEMPLATE = app
DEPENDPATH +=  Proj1 Proj2 Proj3    

include(Proj1/Proj1.pri)  
include(Proj2/Proj2.pri)
include(Proj3/Proj3.pri)

TRANSLATIONS = en.ts fr.ts it.main.ts ja.main.ts nl.main.ts

A script can easily generate this project language file for you.