When creating String resources, as the text is written as node content, we should preserve the format of spaces and line breaks, otherwise the resource below will loose the line breaks:

<sys:String x:Key="Description">This is the first paragraph.

This is the second paragraph of the description.</sys:String>

To fix it, we should write String resources as:

<sys:String x:Key="Description" xml:space="preserve">This is the first paragraph.

This is the second paragraph of the description.</sys:String>