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Weirdly enough, I found a song named "Contra el mundo" that's exactly the opposite of what you're looking for: it's sung by a guy, it does have English words in, it and the tile is never sung word by word, so it's probably not that one: https://www.youtube.com/watch?v=Q7c_6O-9bAI
Also, "Contra o mundo" is not proper Spanish. That "o" sounds vaguely Portuguese, but you must've misheard it. Either that, or the song is in Portuguese.
Edited by NazoIt sounded to me like the words "contra mundo" but I only guessed contra o mundo because that showed up in a web search engine. Thanks for telling me it's not proper Spanish. I am still guessing this song is Spanish instead of Portuguese because the person listening to it was a young man from Mexico.
I already searched the web for this song, but I am limited by the keywords I can use because the song is entirely in Spanish, I only know two words (which I'm not even 100% certain of), and I expect that any description of this song on the web will be in Spanish instead of English so probably will not show as a relevant result to my search.

It's a song I heard this week, but I don't have any idea how old the song is. It's sung by a woman. There were no English words I recognized. I think the entire song is in Spanish. I know very little Spanish, and the only words I recognized were "contra mundo" (possibly contra o mundo or something similar) in the chorus. It's possibly by a Mexican artist because I overheard a Mexican guy listening to this song. I don't know how to describe the genre of the song, possibly a ballad?