First of all, thank you Paul for your response.
We would expect a playlist with songs related to "all" the artists on the catalog, as it actually happens with an artist-radio with 5 different artist seeds:
http://developer.echonest.com/api/v4/playlist/static?api_key=SBLWTA99PCG4NJOYG&artist=u2&artist=carlos+vives&artist=los+pericos&artist=red+hot+chili+peppers&artist=blur&results=100&type=artist-radio
But with the catalog-radio (in the example of the first post) we see that this isn't happening, first because we get a playlist with either all songs in spanish or all the songs in english, and second because it seems that all the songs are related to just one artist from the catalog, the one of the first song in the generated playlist.
As we can notice from your response, it seems that your algorithm is planned to behave this way. We think we can still work with "artist-radios" instead of "catalog-radios", even thought they are limited to only 5 artist seeds :(
Thank you very much again for your help and we would appreciate if you can update this thread if this behavior changes anytime. ;)