The plugin is multilingual at this point, even when it was only in french it still worked on the English client. The lines you mentioned are to do with the list of chat channels the plugin uses, line 22 references a previously loaded script(Language.lua) and that's where the translations are.
If it's saying 'tradcanal' is a nil value(trad being french shorthand for translation, canal being channel) that makes me think you aren't actually running the English locale, or the French or German ones for that matter, and indeed I was able to reproduce the error by manually setting the locale to something other than "en" "fr" or "de". You can check and change which locale you are using near the top right of the launcher, but as far as I know the client only supports the same languages as the plugin. *shrugs*
*It would be best to make the changes while the game is closed though not necessary. You can unload your plugins, make the change, refresh plugins, and then load them again.*
open up the Language.lua file the first line should be:
if it is not, change it to that and that should fix it.
If the line does
match however a not-so-elegant solution should
be to change it to this: