Unless you are not telling us something, the problem should be quite simple. It doesn't matter that you have Arabic or English words. It matters that they are presented in the correct font. You haven't told us enough to make it clear how you intend to discover the correct font.
If you have an ordinary Access text box, the font and reading order are not generally automatic but you can manually set both easily enough. If your problem is that the words in the box SHOULD be shown in an English font or SHOULD be shown in an Arabic font then you have to test the words so YOU can set the font and direction. You would adjust the text box properties .ReadingOrder and .Fontname based on how you want the text displayed.
If instead you have a form of "rich text" feeding the text box then you have to have an indicator in the text box data (.Value or .Text depending on what has focus at the moment) that identifies the selected font. If so, you would have to parse the text to see if you could find the font setting and make your decision accordingly.
If you are using a form of Unicode that contains both Arabic and English characters, you can test the characters because the codes for those two languages do not overlap. The difference between Arabic and English text codes would differ widely enough to be easy to test. But in that case, you would not have to adjust the font (because it is a Unicode font, assuming you could find what you wanted). You would only have to play with reading order.
Also note that there is a setting for the textbox.ReadingOrder property (=0) that if you WERE using detectable Arabic or English characters, it might be able to tell the difference and automatically set the direction for you based on the first character in the box.