DiploList.lua
And how I found it,
Using a good file search tool, search all the xml files for the text "trying to win the game" - it occurs in CIV5GameTextInfos_Jon.xml
Open that file and search for the same text, find the TXT_KEY above it - TXT_KEY_DIPLO_VICTORY_DISPUTE
Now search all the UI lua files for that TXT_KEY - it only occurs in InfoTooltipInclude.lua
Open that file and search for the TXT_KEY, scroll up until you find the method it's in - GetMoodInfo
Search all the UI lua files for that method - it's used in 3 places - DiploList.lua, DiscussionDialog.lua and LeaderHeadRoot.lua
Open each of those and work out if it generates the side lise - looks like DiploList.lus is the one you want - BUT if you change it for that particular dialog it will have the side effect (which may or may not be wanted) of changing it for the other two