That is strange. Most probably something went wrong during installation.
Create a new file called en-GB.plg_cbusercomments.ini place it in:
administrator/langauge/en-GB/ and add this to it.
CB_COMPOJOOMCOMMENT_TITLE="Title"
CB_COMPOJOOMCOMMENT_COMMENT="Comment"
CB_COMPOJOOMCOMMENT_DATE="Date"
CB_COMPOJOOMCOMMENT_NO_TITLE="No title"
CB_COMPOJOOMCOMMENT_NO_COMMENTS="Wow, a shy user! He hasn't left any comments!"
Then it should automatically pick the language file and use those strings.
Ah wait - is the site in german??? If it is in german, then create de-DE.com_cb... file and place it in the de-DE folder in language.
Cheers,
Daniel