I read the campaign guidelines and i still having some doubts:
Good for you, better understand it well and observe how posters post their comments on threads.
-> can i post about any topic in the forum,
Yes you can, provided that your post has quality and really has something it that could contribute on the thread and in the community.
in any place i want and in the language that i want (Spanish - English in my case) or is it ruled by the campaign operators ?
There are campaigns that accepts local posts, but all of it depends on the campaign rules, there is no exemption to that.
All rules that the campaign imposed should be followed, some campaigns might not remove you from the campaign if you post on boards and sections that they don't allow, but for sure they won't count your post. Each campaign has a rule, so better read each before trying.