Hi translators,
When gettext finds obsolete strings in your po file, those are commented out
started with #~.
Meanwhile, Tx seems to remove such strings like follows:
Updating branch refs/heads/master
         to 261e8617acb9f79469110e053f9b431584b71d72 (commit)
       from ae816e5272bed86e660d983e80f9e87ddf865ed0 (commit)
commit 261e8617acb9f79469110e053f9b431584b71d72
Author: Masato HASHIMOTO <
cabezon....@gmail.com>
Date:   Tue Aug 27 18:31:26 2013 +0200
    I18n: Update translation ja (100%).
    
    39 translated messages.
    
    Transifex (
https://www.transifex.com/projects/p/xfce/).
 po/ja.po |   92 +++++++++++++++++++++++---------------------------------------
 1 file changed, 34 insertions(+), 58 deletions(-)
<snip>
-
-#~ msgid "Timeout Interval:"
-#~ msgstr "更新間隔:"
-
-#~ msgid "Show frame"
-#~ msgstr "フレームを表示する"
-
-#~ msgid "Could not create widgets !"
-#~ msgstr "ウィジェットが作成できませんでした !"
-
-#~ msgid "Could not initialize linux backend !"
-#~ msgstr "Linux バックエンドを初期化できませんでした !"
IMHO, we should not keep any additional information for translators
in each po file.
Regards
-- 
HASHIMOTO, Masato