Steps-to-reproduce:

1) Ensure that "US" and "Russian - PC" is part of OS X's "Input Sources" by going to "System Preferences" and adding them under the "Input Sources" tab in the "Keyboard" preference. Also ensure that "Show input menu in menu bar" is enabled so you can quickly switch between the input sources.

2) Create a basic application in Qt, registering "Shift-Ctrl-S" as an accelerator.

3) Start the application with the "Russian - PC" input source already activated.

4) Press the "Shift-Ctrl-S" sequence. Notice that it will not trigger.

5) Switch the input source to "US" while the application is still running.

6) Press the "Shift-Ctrl-S" sequence. Notice that it will still not trigger.

7) Close the application and restart it with the "Russian - PC" input source already activated.

8) After the application has started but before you press any keys on the keyboard, switch the input source to "US".

9) Press the "Shift-Ctrl-S" sequence. Notice that it will now trigger as expected.

I believe that QCocoaKeyMapper is not flushing out the key layouts that are cached whenever the input source changes, causing the issue. In particular, I believe that the commented out code calling QKeyMapper::changeKeyboard() in QCocoaKeyMapper::updateKeyMap(...) is necessary. In particular, the following lines of code:

if (updateKeyboard())

I believe this to be necessary for Qt 5 as well.