the right way how it should be done:
<Page x:Class="Abc.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:lex="http://wpflocalizeextension.codeplex.com"
lex:LocalizeDictionary.DesignCulture="pl-PL"
lex:ResxLocalizationProvider.DefaultAssembly="Abc"
lex:ResxLocalizationProvider.DefaultDictionary="Language">
<FlowDocumentScrollViewer>
<FlowDocument>
<Paragraph><Run Text="{lex:Loc Abc:Language:ParagraphText}" /></Paragraph>
</FlowDocument>
</FlowDocumentScrollViewer>
</Page>