A: Yes. Mods typically affect gameplay values ( gamplay.ini ) or graphics shaders. Audio packs load independently. Just install the mod first, then the language pack.
Because Ubisoft never released an official standalone "English Language Pack" for these versions, the community created their own fixes: The "Language Pack" Files : Fans extracted the ubisoft splinter cell conviction english language pack
The co-op campaign (Voron/Archer) uses separate audio files. If English voices work in single-player but die in co-op, your pack is missing Coop_EN.fsb . Fix: Ensure your language pack includes the COOP subfolder assets. A: Yes
HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Ubisoft\Splinter Cell - Conviction (or HKEY_LOCAL_MACHINE\SOFTWARE\Ubisoft\... on older 32-bit systems). Values to Edit: ubisoft splinter cell conviction english language pack