Ostensibly[Officially], her nationality is Japan but everyone doesn't consider her as a genuine Japanese because she doesn't speak Japanese at all.

查看翻譯

Officially is better in context. It means there are documents to support the fact that she's Japanese, but most people still believe she isn't because she doesn't speak the language.
Ostensibly means she is supposedly Japanese but may not be. But her nationality IS Japanese. There is no supposition to be made.
The complete sentence:
"Officially, her nationality is Japanese, but not everyone considers her to be genuinely Japanese because she doesn't speak the language at all."