Have you not got an English version of Homeworld 2?
To download the english version follow these steps:
download English.zip;
place the files EnglishSpeech.big and English.big in your ..\homeworld2\data folder;
rename them like your original files, (ex. if you have an italian version rename them ItalianSpeech.big and Italian.big),
make a back up of your original files.

Do you have any problem with the game installation?
Be sure you have upgraded your Homeworld 2 to version 1.1;
clear your ...\Homeworld2\Data directory from any other installed mod/addon, your ..\Homeworld2\Data directory must contain only files/directories listed below:
-Animatics
-Homeworld2.big
-Music.big
-English.big
-EnglishSpeech.big
install Complex in your Homeworld 2 installation folder;
run Complex on your desktop.
If you have Complex in the big format, copy the file Complex9x.big in your ...\Homeworld2\data installation directory (example C:\Program Files\Sierra\Homeworld2\data\Complex9x.big);
open your shortcut to Homeworld 2 (in your desktop or whatever you have it) and type -mod Complex9x.big at the end of the path (ex. "C:\Program Files\Sierra\Homeworld2\Bin\Release\Homeworld2.exe" -mod Complex9x.big);
note the case sensitive and space sensitive nature of this command line.
If you still have problems, try to post on the Complex forum

Do you want host Complex in your web site?
be sure you have a link to www.homeworld2complex.com;

Other technical issues
Screen resolution: you can use any resolution of your choosing, such as those listed below. Simply cut and paste from the text below into the Homeworld2 Complex shortcut properties,
(ex. "C:\Program Files\Sierra\Homeworld2\Bin\Release\Homeworld2.exe" -mod Complex9x.big -w 1920 -h 1200),
note the case sensitive and space sensitive nature of this command line.
You can try using this method to run in any one of the resolutions listed below or others that may not be listed;
16/10 aspect ratio:
1600x1050 -w 1600 -h 1050
1920x1200 -w 1920 -h 1200
2560x1600 -w 2560 -h 1600
Graphic quality: if you want to enjoy the full visual experience of Complex, you will need to set these values to high (in game):
Enable V-Sync: ON
Shadows: ON
Hyperspace Quality: HIGH
Detail Geometry: ON
Damage FX LOD: HIGH
Lighting LOD: HIGH
Geometry Scaling: OFF
NLips: OFF
Also you can enable Antialiasing and Anisotropic filtering, if your video card support them:
right click with your mouse on the desktop -> properties -> settings -> advanced -> find these settings in your video card specs and change it as you need.
Please ensure you have the latest video drivers before running Homeworld 2 Complex.