There are lots of ways to contribute, depending on your interests, like:
-The spanish translation needs improvement. See wiki for details.
-Artwork, Modeling, Mapping if you have 2D/3D design/graphics skills.
-Create music/sounds if you have experience with it.
-Test play the game and maps, esp. new features, write bug reports, give feedback.
-Participate in this forum, discuss and help to answer questions.
-Help improve the wiki.
-Tell your friends about UFO:AI ;-)
-...
Theres also lots of information on the wiki about contributing...