To create the 'all_language.nsh' file you need to do a few things first.
Remember to use the UNICODE version of NSIS or it will not work. If you try to run it with the ANSI version it will halt so do not bother to try.
You also need Polib - Library for python to manipulate PO files.
To get the POT file run:
python build_gettext_catalog_pot.py -i AirDC_installscript.nsi -o EN_Installer.pot -p "AirDC++" -v "1.0" -l "English"
and you will get 'EN_Installer.pot' to upload to Transifex.
Download the PO files from transifex (Download for use).
You will then have one file for every language. This is the Swedish file 'for_use_airdcpp_en_installerpot_sv_SE.po', but you need to rename it to 'sv_SE.po'. Do the same for every PO file and put them all together in the 'pofiles' folder. You can read in the 'build_locale_nsh.py' what name you need to use on the PO files.
We don't want to use all the text from 'AirDC_installscript.nsi' so we use the dummy_file.nsi file instead. To get 'all_language.nsh' run this:
python build_locale_nsh.py -i dummy_file.nsi -o all_language.nsh -p "pofiles/" -l "English"
You now have all translated strings in 'all_language.nsh'. It will be included into the AirDC_installscript.nsi when you compile it.