I have not noticed it prior to b15, but it could be there earlier too. If I had the keyboard in a different language prior to opening the Master Password Screen I can not switch it back. The language button is greyed out.
Could I get you to try updating to the latest beta to see if the problem still persists? I am able to switch between languages on the master password screen in beta 16. Hopefully that resolves the issue for you.
If the issue remains after updating, could you provide me with the following so that I can try to better reproduce the issue on my end:
I am running b16.
1. HTC One Sprint
3. English and Russian
4. This is HTC Sense keyboard
I don't have an HTC device to test against, but I believe that the key difference here may be the fact that you are using the HTC Sense keyboard. I will pass this on to our development team so that they can look into it. Thank you for the feedback!