。
A Japanese person would never do something like that.
Literal
Japanese person [conditional-なら] that-kind-of thing [topic-は] absolutely do-not [conjecture-でしょう].
Two useful constructions. ~なら is the topical conditional ('if we're talking about/as for') — 日本人なら frames 'Japanese person' as the hypothetical subject. けっして ('absolutely, by no means') is an emphatic adverb that always pairs with a negative predicate — similar to English 'never' or 'by no means.' The whole structure expresses a cultural generalization.