On R2 discs subtitles are always player-generated, whereas they are often burnt in with R1 DVDs. So the alien languages are subtitled in a default subtitle stream that is automatically active.

However, when I watched the films yesterday this subtitle stream was glitchy... in a few scenes the subs didn't appear, and I had to go to the language menu and select "no subs", which reset the subs back to what it should be. Then they showed up. I don't know if it's a disc authoring problem or what; but yeah, I can understand if people don't see the alien language subs sometimes.