The static text of custom blocks can be localized during design by referencing custom messages rather than typing literal text.

The Screen Designer tool allows you to reference custom messages by key. If you are editing block XML files manually, you can use tags tc:message and tc:messageParam to achieve the same effect.

Text that is added to a block by entering literal text in the Screen Designer, or by using the out tag with literal text, will not be localized. It will always by rendered as it was originally entered.