I tried to compile the engine and indeed it is stopping in the install phase. Looking at the spec (you can open it, it's a text file), it requires two language tar files that I don't have. Maybe you can adjust the `tar -xvfj ${SOURCE1}` line to extract only what you need to for your language. You'll have to modify the for loop just after.

When a package is built and you hve RPM in the RPMS directory, install the development package in the SDK itself with:
sb2 -t SailfishOS-3.0.3.8-i486 -R rpm -U RPMS/okb-engine-devel-0.6.19.rpm
(adjust the name of the package of course, you'll need to install also the okb-engine one I guess, so put the two rpms on the same line)