Using the level editor, put a Sign Area around the sign. Select the sign area, and on the right hand side, click the "Area" properties tab. Specify a text category and text entry (these will be the category and entry that you write in your .lang file). Add the category and entry into your .lang, then your done! I'd suggest making the category called "Signs", then use different entries for each sign.
Example: Category is called "Signs", and Entry is called "PrisonB"
<LANGUAGE>
<CATEGORY Name="Signs">
<Entry Name="PrisonB">Prison section B</Entry>
</CATEGORY>
</LANGUAGE>