See Developer's Guide page in the URL of this Issue. This page says concerning the 
Title of a menu or submenu:
"This property supports localization: The default string, which is used when 
OpenOffice.org cannot find a string definition for its current language, uses the 
value element without an attribute. You define a string for a certain language with 
the xml:lang attribute."
The same text appears also for toolbar buttons.

This is no longer true with 2.4 RC6. I can't say when it regressed.
I tested a simple add-on with:

<prop oor:name="Title" oor:type="xs:string">
  <value>Test lang=none</value>
  <value xml:lang="fr">Test lang=fr</value>
  <value xml:lang="de">Test lang=de</value>
  <value xml:lang="es">Test lang=es</value>
</prop>

I configured my OpenOffice with the italian (it) langpack.
Expected : Test lang=none
Obtained:  Test lang=de

I replaced the add-on with another using this:

<prop oor:name="Title" oor:type="xs:string">
  <value xml:lang="fr">Test lang=fr</value>
  <value xml:lang="en">Test lang=en</value>
  <value xml:lang="de">Test lang=de</value>
  <value xml:lang="es">Test lang=es</value>
</prop>
I tested again with the italian langpack.
Expected : not defined in spec
Obtained : Test lang=en

I got the same results also for the Toolbar title.