エリナ:窓の外に見えるの皆シットル? Erina: Couldn't the rest of people look out of the window?
タダシ:そうだろう たぶん
Tadashi: Yeah, maybe.
CONTEXT: After taking a flight and stopped in a station Tadashi is looking at a bunch of people in that station, then Erina ask him -what I guess- is something like "the rest of people did look out of the window?" then Tadashi answers "yeah, maybe", but I'm not sure how 皆 is related to シットル.
(rough translation attempt)
タダシ:このディスク。。。やっぱ このフライトレコーダーのじゃないかな
Tadashi: This disk... It seems that it didn't belong to the black box.
エリナ:うーん。。。 でも何でわざわざわかりにくい所へかくしてあるわけ?
Erina: Yeah... but why they wanted to be this disk hard to read? did they have something to hide?
タダシ:フライトレコーダーのだったとしたらさ こう何か とくべつなひこうきろくが のこされてるとか! (what does こう何か mean?)。。。 っていうのは?
Tadashi: If this were a black box (...) this special register left here! Don't you think?
エリナ:タダシにしては まあまあの せんね
Erina: Being mad is not like you.
タダシ:あのさ。。。!
Tadashi: Sorry...!
CONTEXT: after looking inside a war robot, Tadashi and Erina founded a strange disk and an empty black box, then Tadashi says that that disk may not be for the black box, then Erina ask about the purpose then Tadashi gets mad. I can't relate the third sentence because of the こう何か.
There is only one place...
No. 見えるの and まあまあの線ね are not dialectal, either. It's not まあまあ+のせん+ね but まあまあ+の+線+ね. If you don't know/understand the phrase 「まあまあの線(だ)」, I recommend you check the meaning/usage of 線 in the common expression 「いい線(を)行っている」ejje.weblio.jp/content/いい線いってる