Senku stared at him a moment, considering his words, but it wasn't because of what Siebren asked of course, he was trying to parse out potential meaning.

He hadn't gestured, he hadn't really done anything at all to help him understand.

What he's speaking is...Germanic, certainly, beyond that, he's got nothing. His brow was furrowed. His inflection, maybe, told him he was asking a question? Maybe.

"Uh. Okay. I didn't understand that. Just...Hm." He was considering some kind of potential solution. Unlike Siebren, he nearly always defaulted to Japanese since they had translators.