The Arabic and the Hebrew codepages are bilingual (i.e. they contain English and Arabic or English and Hebrew characters). When either of these codepages is active, the system fonts support all the characters in the relevant codepage. This means that the system fonts are bilingual, for example "Courier" would contain both English and Arabic/Hebrew. shapes.
This means that applications do not need to change fonts when switching between Arabic/Hebrew and English characters.
Note: Arabic and Hebrew postscript (Adobe Type 1) fonts are created using font specific encoding. This means that the value used for the codepage of these fonts is "65400" rather than the national codepage. Applications that specify the usCodepage field when creating a logical font should use this value in order to get a match for the Bidi postscript fonts.