Talked with Grum shortly after about it. There's a new code for this (not in 1.7.4 yet, could be in the latest snapshot(s) though, I didn't check), which should fix this issue. Can be marked as closed.
I honestly don't see how that is an issue. Fact is that getStringWidth returns an incorrect string length, which will cause any right/center aligned bold text to not display properly.This applies to both, vanilla and modified by whatever 3rd party tool.
Can confirm on a Slovenian keyboard. And I may have tracked it down. Minecraft checks inside GuiTextField.onKeyDown for CTRL+A/X/C/V explicitly.
public static boolean isKeyComboCtrlV(int key) { return key == 47 /* V */ && isCtrlKeyDown(); }
A possible fix would be to check for !isAltKeyDown() as well.