when I switch to another keyboard language on iOS 9 using a Bluetooth keyboard, VoiceOver still behaves as if the language was not changed until VoiceOver is turned off and on again. However, this problem affects only the VoiceOver related features such as VoiceOver keyboard shortcuts, keyboard help, or search in window items activated with VO + F. Otherwise, for example, in text fields or when using app keyboard shortcuts, the change of the keyboard language is respected properly and immediately.
I will give an example of how not knowing about this bug made me think for some time that some of VoiceOver keyboard shortcuts are randomly not working:
If Single-key QuickNav is enabled, you can navigate by heading levels using the number keys 1 through 6 on the US English keyboard. However, on the Czech or French keyboard, you have to press the number keys together with the Shift key. And at some moment, I switched the keyboard language from Czech to US English. Later, when I wanted to navigate by heading levels, I thought the number keys shortcuts stopped to work because I didn't know that due to this bug they still need also Shift to be pressed to activate the shortcuts.
I've already reported the bug to Apple at firstname.lastname@example.org but if you find the bug annoying, please do so as well.
I've encountered the bug in iOS 9.3.3 on iPhone but it's possible the bug exists also in prior versions.