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.