can't change the keyboard layout

Real keyboards have always worked in Mir. The OSK issue you describe is Unity8 and/or other downstream projects.

When you say "change the keyboard layout", do you mean switching between a real and OSK, or just using a real keyboard with different layouts?

Using the real keyboard with a different layout, I've an azerty keyboard on my laptop and /etc/default/keyboard contains

"XKBMODEL="pc105"
XKBLAYOUT="fr"
XKBVARIANT="oss"
XKBOPTIONS="""

xorg picks those value and let me type in azerty, but when using unity8/mir the layout is qwerty. Out of respecting the default system value the server also needs to provide an API for clients to get/change the active layout since that's something quite some user need (especially when dealing with input methods)

Status changed to 'Confirmed' because the bug affects multiple users.

from where unity or mir get to know the layout? would it help to change some file?

This bug was fixed in the package unity8 - 8.12+16.04.20160323.2-0ubuntu1

---------------
unity8 (8.12+16.04.20160323.2-0ubuntu1) xenial; urgency=medium

  [ Michael Terry ]
  *
  * Fix setting OSK layout in OOBE wizard
  * Stop the left edge tutorial from being shown when the bottom edge
    tutorial wants to be shown after receiving a call.

  [ Albert Astals Cid ]
  * Filter implementation with Option Selector filter as the only filter
  * Get the engine from the parent context
  * Implement the API changes due to filters
  * Match design visuals for the new dash navigation/page header popup
  * New Dash Navigation UX
  * Range Input filter
  * Value slider filter
  * Workaround SDK ListItem bug

  [ Albert Astals Cid, Lukáš Tinkl ]
  * Fix various tests

  [ CI Train Bot ]
  * Resync trunk.
  * Update translation template

  [ Lukáš Tinkl, Michael Terry ]
  * New first-run wizard for the phone (part of the OOBE effort) (LP:
    #1536688, #1536617, #1543790, #1555222)

  [ Lukáš Tinkl, Michał Sawicz ]
  * Keymap switching support (LP: #1524400, #1412492, #1491340)

  [ Michael Terry, Michael Zanetti ]
  * Redesign the first-boot edge tutorial

  [ Michael Zanetti ]
  * disabling some tests that are flaky in adt but can't be made fail on
    our machines

  [ Michael Zanetti, Michał Sawicz ]
  * Change the behavior alond with the switch in indicator-display (LP:
    #1538591)

  [ Michael Zanetti, Michał Sawicz, Nick Dedekind ]
  * Add support for low shell chrome. (LP: #1535397)
  * Sidestage load/unload redesign (LP: #1544479)

 -- Michael Zanetti <email address hidden> Wed, 23 Mar 2016 09:53:17 +0000

Why is this bug considered as fixed? I have a BQ M10 Ubuntu tablet updated and it is still impossible to change USB keyboard layout.

Update: With BQ M10 Ubuntu OTA 13, it is now possible to select the keyboard layout in:
Settings/Language & Text/Keyboard Layouts/External keyboard (option displayed when an external keyboard is plugged)

However, the change is not saved. So, it still does not work...