WeiDU uses the tra files specified in the LANGUAGE section as defaut tra files to look for text references. If you have a dialog or script that reference @112 and that reference is not present in the tra file associated to the dialog or script (either implicitely, because the file has the same name, or explicitely with USING ...), WeiDU will also look for @112 in the two tra files stated in the LANGUAGE part.
WeiDU uses the tra files in LANGUAGE in reverse order. Some mods (Rogue Rebalancing, for instance) use that form:
This allows adding new things in the mod (English/Setup.tra) without preventing installation in French if a text is not translated yet. The english text will be used, but the mod will be installed.
If you're going to reference several tra files in LANGUAGE (for the same language), you should ensure that they use a different range of @nnn to avoid any kind of conflict.
If your dialogs are all starting at @0 or @1, it is also good practise to make all your tp2 text reference start at @10000, for instance (provided you don't have any dialog reaching that value), in order to be sure that a missing @nnn somewhere is discovered, instead of replacing a missing dialog text by a string intended for the tp2 (objet description or whatever).