Hey, firstly, I have no scientific research on this matter, just my personal opinion and some collected info from Spanish speakers.
So I'm inclined to think that Spanish and Slavic/Baltic languages have many similarities (in vowels maybe?). Certain people from Spain actually stated that speakers from Slavic regions can imitate Spanish accent rather easily, and generally are more understandable than the other foreigners. Any Spanish speaking person can confirm this?
I myself, despite not speaking Spanish at all, and being from Baltic region, don't have much trouble with listening in Spanish language (though this might have been an effect of Spanish dramas I had been watching on TV as a child). Also, the words don't seem to be very difficult to pronounce and I feel that I can get them right (although a Spanish speaker would have yet to confirm this). Compared to French, where all the vowels and consonants are like tongue twisters to me, Spanish just seems to be much easier.
So any ideas? Is it true that Spanish (and maybe Portuguese) and Slavic/Baltic languages have lots in common phonologically?