。
It's not as if she fully understood my English.
Literal
She [topic-は] my English [object-を] completely understood [not-the-case-that-わけではなかった].
~わけではない (or ~わけじゃない in casual speech) is a hedging negative pattern: 'it's not the case that, it's not as if.' It denies a possible interpretation while leaving room for partial truth — here, 'she didn't fully understand, but she may have understood some.' 完全に ('completely') is the adverb being denied — together with わけではない, this gives a precise nuanced statement: total understanding didn't happen. This grammar is essential for nuanced Japanese; bare negatives often overstate.