Take this example.
I have English article "A". I want to Apply German translation "B" to article A. However, even though I typed "B" in German, I might have accidentally let it save so that the wiki thinks it's English. In the case importing from NuWiki, this probably happened automatically. VaultWiki won't let you claim that "B" is German when it thinks it's English (allowing this freely could potentially corrupt page output). To set VaultWiki straight, you first need to visit "B" and Apply it as German.
For an improved workflow, it's probably best to set all of your German translations from each German article, rather than from the English one. This way you can change its language and set the English translation at the same time.