There are many ways, depending on how wide a scope you want.

If there is one specific window that you want to follow right-to-left layout rules, then you pass the WS_EX_LAYOUT­RTL extended window style when you create the window. This extended style is inherited by child windows, so once you set a top-level window to have right-to-left layout, all child windows will have it, too. To block the WS_EX_LAYOUT­RTL extended style from being inherited by child windows, pass the WS_EX_NO­INHERIT­LAYOUT style when you create the parent window.

Sidebar: If you're calling the Message­Box function, then you don't directly control the styles of the top-level window. But there's a weird back-channel way to specify that you want the message box dialog to have the WS_EX_LAYOUT­RTL extended style: Begin the lpText string with two U+200F characters. Then again, instead of Message­Box you should be using the Task­Dialog­Indirect function which not only lets you customize the text on the buttons, but also lets you pass the TDF_RTL_LAYOUT flag to indicate that you want the dialog to be laid out according to RTL rules. (And as an aid to porting, the Task­Dialog and Task­Dialog­Indirect functions implicitly turn on the TDF_RTL_LAYOUT flag if they find that pszContent is a pointer to a string—not a MAKE­INT­RESOURCE—which begins with two U+200F characters.) End sidebar

If you want right-to-left layout rules to apply to all top-level windows in your process, you have two choices. You can either do it programmatically or declaratively. (Similar to how you can specify DPI-awareness either programmatically or declaratively.)

The programmatic way is to call Set­Process­Default­Layout(LAYOUT_RTL) from your application. The declarative way is to insert two left-to-right marks (U+200E) at the beginning of the File­Description version resource string of the executable.

Note that the caveats which apply to changing the process DPI awareness programmatically also apply to changing the default process layout programmatically: Code which calls Get­Process­Default­Layout will see the default at the time of the call, even if some code later on calls Set­Process­Default­Layout to change it.

Note also that it really is the application's call whether its default layout is left-to-right or right-to-left. A DLL shouldn't decide on its own to change the process default layout, at least not without coöperation from that process. If you are a DLL and you want to create a specific window with right-to-left layout, you should use the WS_EX_LAYOUT­RTL method so that your decision applies only to your DLL's windows. (Otherwise you're using a global setting to manage a local problem.)

Bonus chatter: Why isn't the default layout specified in the manifest like DPI-awareness? Because RTL support was added in Windows 2000, which predated application manifests by several years. If the feature were invented today, the manifest would be a much better place for declaring it.

Update

¹ Commenter SCB pointed out that there is indeed a flag to specify that you want RTL layout on your message box: MB_RTLREADING. If that flag exists, then why also have the U+200F back-channel?

Answer: So that translators can mark a string as requiring RTL treatment without having to go back and make code changes.