Unexpected results when attempting to assign custom names to higher mouse buttons via custom lang files

The bug

When using a custom resource pack to assign custom names to mouse buttons 4 and up, strange results can be seen in the controls menu.

How to reproduce

1. Download and apply the attached resource pack
2. Bind controls to mouse buttons 4, 5, 6, 7 and/or 8
3. Quit the game
4. Reopen the game
5. Navigate back to the controls bound to these mouse buttons
6. Disable the resource pack
7. Check back on the controls once more

Results

When the resource pack is first applied, the controls bound to mouse buttons are named "Mojira 4", "Mojira 5", ... This is despite key.mouse.4, key.mouse.5 and so on having explicitly defined names in the en_us.lang provided in the resource pack.

When the game is exited and loaded back up again, we correctly see the names "Mouse Back", "Mouse Forward" and so on assigned to these buttons.

When we disable the resource pack, the button names are reset to their translation strings: "key.mouse.4", "key.mouse.5", ...

Expected results

While the resource pack is active, the custom names for each button should be displayed. While the resource pack is not active, it should fall back to the default "Button 4", "Button 5", ... names. Whether or not the resource pack is active from the time the game is first loaded, or applied after loading, is irrelevant. The cases "Mojira 4", "Mojira 5", ... and raw translation strings should never be visible.