STEPS TO REPRODUCE
1.  Make sure your system uses two input languages with different alphabets (i.e. you switch input languages betweeen English US and Cyrillic/Arabic/whatever.)
2. In Settings - Mouse - Configure extra buttons -> Add Binding: add binding of side mouse button to Ctrl-V and the other one to Space
3.  Alternatively, add the following section to  .config/kcminputrc and reboot:
ExtraButton2=Key,Space
ExtraButton2=Key,Ctrl+V

Essentially, we mapped paste shortcut to one of the mouse side buttons, and spacebar to the other side button.

OBSERVED RESULT

Clicking of the side button invokes paste only if English is active input language.  When the other input language is selected, the mouse side button  acts normal (i.e. back or forth)

The spacebar binding though works regardless of the selected input language.

EXPECTED RESULT

The results should be consistent. I.t. either all button work regardless of the input language or they don't.
Personally I think they should send commands as programmed, disregarding the active input language. 
To my knowledge, there's now way to use a scancode instead of actual letter C, which could remediate the issue.

That's the way other programs work, i.e. Ctrl-c/v  work as intended regardless of the active input language.

Unfortunately, Non-English languages exist and the users are forced to switch between input languages, so the developers should probably test them.