When I try to install the german language support as an extension I get the following error: "Mail/News could not install this item because "install-f3n..rdf" (provided by the item) is not well-formed or does not exist. Please contact the author about this problem." I used the de.xpi from ftp.mozilla.org/pub/mozilla.org/thunderbird/releases/1.5/linux-i686/xpi/
Report this to the TB localization team, Gentoo only distributes the language packs.