While the last few posts on this thread have been devoted to just one song in Spanish, there might be a bit more to be learned in taking a more general approach.

Personally, I had exactly this problem last year when I sang with the NZ Anthem Choirs for the Rugby World Cup. As a choir singer for four of the matches, I had to learn anthems in "foreign" languages for two counties - Wales and South Africa. And not only learn the words, but the bass parts as well.

In both cases I found the only way to do it was to write down the words phonetically. We had been given the music and words for each anthem, but written Welsh misleads, as it is not pronounced in the same way that you would if reading English. And the South African national anthem contains lines in five different languages, three African tongues, Afrikaans (basically Dutch) and English. The latter I two I could cope with, since I speak some Dutch and I am a native English speaker. The African languages again had to be learned phonetically. Fortunately, the NZ Choral Federation had anticipated this problem and they provided pronunciation guides for all the anthems on their website.

In both cases, I also found that looking up the Wikipedia pages for each anthem was useful as they gave an English translation, so I was able to get the idea of what was being sung, which in an indefinable way helped to get the emotions across.

Having got the phonetics and the music, it was just down to hard work and practice. The Welsh anthem "My henlad..." is a glorious song to sing anyway and the bass part just fell into place. The African words seemed to make a kind of sense and both anthems came out beautifully when we sang them on the pitch.

Fortunately, Wales got into the quarter-finals so I got two chances to sing their anthem. A great experience which improved my sight-reading considerably.

The general point seems to be that if you want to sing anything in a language foreign to you, do a lot of work on pronunciation, understand what you are singing about and then go for it!