Looks like it got the dark elf and "I live in" parts, but "fitness room in Thailand" is probably what is commonly referred to as "neural network hallucination."

AFAIK the premise of NNMT is that it doesn't try to do much in the way of parsing, but tries to "learn" the association between phrases in the two languages by matching of the training data, so if it sees "fitness room" and "Thailand" and that happened to somehow align with an occurrence of ほくとうのしまのどうくつ , that's what it will think they translate to.

You can see some interesting "probing of Google Translate's guts" here:

https://languagelog.ldc.upenn.edu/nll/?p=35120

Who knows how it came with up with that crazyness.

The original translates to something along the lines of "Dark elves live in the caves on the islands in the north east".

Maybe it took "north east" to mean Thailand and instead of separating しま and の it just took it as one block of text しまの (Shimano) like the bike company, and therefore fitness and どうくつ is cave which is in a way a type of room but that is one hell of a tortured translation.

Good times.

One person's "tortured translation" is another machine's "idiom and poetic metaphor".