I’m a newbe in this world of modding, but I love BMS idea. I wonder if you are going to use the original hl1 files for speeches, dialogues and screen texts. If that is not the case, I offer myself to make a sensible spanish adaption for this mod. You can contact me by e-mail.
The devs have hired voice actors to recreate original lines and read some new ones.
check out the website and the forum rules while you’re at it.
Hi udogi, you might want to check out this thread: https://www.blackmesasource.com/forums/showthread.php?t=70
Also, udogi, I am afraid the Spanish translation of the mod has already been covered, as far as subtitles are concerned.
If you want to organize a dubbing project, though, then things will become a bit more complicated. There have been comments about the idea of dubbing, but there seems to be a certain rejection towards the idea.
Sadly, you won’t have chance to do that. 
But i may do the Chinese dubbed version. but not by myself.