Oh, I should have posted a link... you can see the GlobalStrings.lua file for all locales here:
https://github.com/phanx/wow-globalstrings
Unfortunately it looks like there isn't a global string for "Thunderforged". But, I'm fairly certain that all the other possibilities for that line (Heroic, Raid Finder, is there anything else?) do have global strings, so if you see green text in GameTooltipTextLeft2 and it isn't ITEM_HEROIC or RAID_FINDER, then it's probably safe to assume it's "Thunderforged".