There is a strange behavior when trying to create string which contains a Hebrew letter and a digit. The digit will always be displayed left to the letter. For example:
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The unicode characters "RTL mark" (U+200F) and "LTR mark" (U+200E) were created precisely for this purpose.
In your example, simply place an LTR mark after the Hebrew character, and the numbers will then be displayed to the right of the Hebrew character, as you wish.
So your code would be adjusted as follows:
string A = "\u05E9"; //A Hebrew letter
string LTRMark = "\u200E";
string B = "23";
string AB = A + LTRMark + B;