Do you need to know the language that an opera is in?

Most viewers will get a copy of the libretto in English when watching or listening.

If you weren’t already familiar with the opera, scene by scene, you used to have to bring or buy a booklet containing the lyrics (called the libretto) and translation. These days, when you attend an opera in person, you may not have to rely on a hard copy. There may be a screen above the stage that has a running display of the lyrics while they’re being sung, even sometimes when the libretto is in English. These are called supertitles (“super” = “above”).

So it is nice if you know the language, and it’s especially nice to be familiar with the words of the big arias and choruses, but it’s not necessary when there are supertitles.

You can also enjoy the music without translation, and when you see a performance you can follow a lot of it just by the action onstage—unless, of course, it’s one of those zany, illogical comic operas with people disguised as other people and climbing in and out of windows or hiding behind bushes, trying to deceive someone. But don’t even bother to follow those—just relax and enjoy them.

When you watch one on video, the lyrics are probably going to be included as subtitles.

I don’t speak or understand Klingon, but I would jump at the chance to see ” ‘u’ ”.

Not necessarily. While I’m not an opera fan I listen to lots of music where I don’t understand a single word of the lyrics. To me it matters little in most cases as, with a few exceptions (like Woody Guthrie, John Prine), I find the lyrics themselves aren’t of primary importance but rather are simply something to hang the vocals on. Then there’s someone like Lisa Gerrard – she’s a masterful vocalist who typically doesn’t even have lyrics, but rather uses a kind of glossolalia (“speaking in tongues”). The Icelandic band Sigur Ros often employ a similar wordless technique to their vocals.

Don’t worry about the language now. You can look up the plot to any opera, and read translations of the libretto. Concentrate (listen) on the “tunes” that catch your ear. Every opera will have its share of showstopper arias, and you will be surprised at how many of the “tunes” have bled into commercials, cartoons, etc. There’s tons of stuff you will recognize like the “toreador song” or “Glitter & be Gay”; loads of spectacular stuff built around showing off the voice. And most important, when you come across a version of an aria you find appealing, be sure to listen to a variety of different singers attacking the challenge. Thank God these days for you tube.I envy you in your upcoming adventures.

No, you don’t need to know the language being sung to enjoy an opera, in my opinion. As @Jeruba says, modern productions will feature a translation in “supertitles” so one can follow the plot. When I listen to an opera on CD or vinyl, I like to follow along with the libretto. I look at both the Italian (or whatever the language may be) and the English rather than look only at the translation. I’ve learned some Italian this way. :) In opera, the translation may be more important than say in a foreign language pop song, but in either case I enjoy the sound of the language even if I can’t understand it.

No. Truth is, opera is literally unintelligible even if you are a native speaker.