Nope, that’s not a mistranslation! But the joke is *because* Reiner is gay. That’s like, why that part is supposed to be funny. It’s also a repeat of the same joke that was made earlier in the manga.
As for confirmation of Reiner being gay, it’s only mentioned once, in chapter 38. It’s during the part in which Reiner interrupts Ymir sneaking around inside the tower at night.
REINER: ユミル… 何してんだ？ (Ymir… What are you doing?)
YMIR: 何だよレイナ―… 夜這いか？ (What are YOU doing, Reiner… Trying to sneak a grope at night?) **Note: Ymir accuses Reiner of trying to 夜這い which is a word that literally means a man trying to sneak into a woman’s room at night to have sex with her.
YMIR: 驚いたな… 女に興味があるようには見えなかったんだが… (I’m surprised… You never looked like you were the type of guy interested in women…)
REINER: ああ… お前も男に興味があるようには見えんな。 (Yeah… Well, you’re just like me, since it doesn’t look like you have any interest in men.)
YMIR: は… (Yeah…)
And also the way that Ymir smiles at Reiner when she says that last line shows that she’s agreeing with him.
I mean there’s really no other way to interpret that passage other than Reiner confirming that he’s gay, especially the way that he says お前も (“you too” or “you’re like me”) to Ymir. When saying specifically that she has no interest in the opposite sex. In the context in which it is abundantly clear to everyone in the 104th (and even confirmed on the official SnK website) that Ymir is in love with Krista. So there’s not even wriggle room to interpret this as Reiner saying that Ymir is uninterested in the opposite sex because she’s uninterested in romance period, it is clearly Reiner observing that Ymir is uninterested in the opposite sex because she is obviously in love with another woman which means that the parallel that he’s drawing to himself can only have one meaning too. Ymir is “just like” Reiner because she is attracted to someone of her own gender, and the implication there is pretty crystal clear.
I guess if you really want to do some mental gymnastics you could interpret that passage as Reiner saying something other than confirming that both he and Ymir are gay, but… Occam’s razor and all. Also that’s the only thing that makes sense for Reiner to be saying to Ymir in that context, considering that a) it’s obvious to EVERYONE that Ymir has a thing for Krista and b) Reiner is trying to extend an olive branch to her, hence him revealing that bit of personal information about himself - and Ymir smiling and relaxing when she hears it. Mission (temporarily) accomplished.
ETA: Added just because I’m tired of repeating this every time somebody asks me about this line again: For the record, “I’m not interested in the opposite sex” is also something that frequently shows up in Japanese books, comics, and film as code for a character saying that they’re gay, so there you go.
ETA again: Another ask about the Krista thing.