EDIT:
IF you just want to change Text Alignment then there is always ToolStripItem.Alignment or ToolStripItem.Padding to try...
The feature you are using is meant for localization and can support mixed content...
The way you use that feature seems more like an abuse... since Windows needs to make sense of "mixed content" of which you provide an extreme (no arabic at all).
I find it always hard to be sure that such behaviour though unexpected and unintuitive is really a bug...
Rendering logic for BiDi text is rather complex - for example when you have an exclamation mark somewhere in text which should be rendered RightToLeft it can lead to reversing the direction depending on the implementation...
For some insight see http://www.unicode.org/reports/tr9/
As it seems .NET is not fully compliant with the Unicode BiDi algorithm... there is even library that tries to implement it see http://sourceforge.net/projects/nbidi/