What is use of android:supportsRtl="true" in AndroidManifest xml file

Declares whether your application is willing to support right-to-left (RTL) layouts. If set to true and targetSdkVersion is set to 17 or higher, various RTL APIs will be activated and used by the system so your app can display RTL layouts. If set to false or if targetSdkVersion is set to 16 or lower, the RTL APIs will be ignored or will have no effect and your app will behave the same regardless of the layout direction associated to the user's Locale choice (your layouts will always be left-to-right).

The default value of this attribute is false.

This attribute was added in API level 17.

(Source: http://developer.android.com/guide/topics/manifest/application-element.html)

if you are building an app in Arabic, Urdu, Hebrew, etc... or any language that is written from right to left you should set android:supportsRtl to true, that's how you tell the layout to be from right to left, and The default value of this attribute is false.

From Android API-Guides:

(developer.android.com/guide/topics/manifest/application-element.html)

Declares whether your application is willing to support right-to-left (RTL) layouts.

If set to true and targetSdkVersion is set to 17 or higher, various RTL APIs will be activated and used by the system so your app can display RTL layouts. If set to false or if targetSdkVersion is set to 16 or lower, the RTL APIs will be ignored or will have no effect and your app will behave the same regardless of the layout direction associated to the user's Locale choice (your layouts will always be left-to-right).

The default value of this attribute is false.

This attribute was added in API level 17.