Spanish!
I'm learning Spanish myself, but I do spend quite a bit of time in Spain. Portugese is very similar to Spanish when you see it written down, but sounds very different when you hear it. Even so, once you get the pronunciation right you should make the crossover quite easily.
The difficult bit for me is getting my ear in... understanding what native speakers are saying. If you are dealing with native speakers a lot, then you already have a natural advantage.
For listening to the language I find the Yabla app very useful. I also watch Spanish tv for free... google RTVE.
Good luck.
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Steve J
Unkind tongue, right ill hast thou me rendered
For such desert to do me wreak and shame