This is an attempt at a patch for the bug:
http://forums.lotro.com/showthread.php?365323-Turbine.PluginData.Save-in-german-%28and-french%29-version-of-client
OK, it uses a dirty trick and it is poorly written (I just discovered lua yesterday) but it seems to work quite well for me... At least, it permits to used all these wonderful plugins created by the community until a real fix for this bug is released....
Tested (on my french client) with: tonicbars, buffbars, manager, quickslotbar (it should work with pretty much any plugin)
HOW TO USE IT:
1 - copy the VindarPatch folder in the lotro \Plugins as you would do for any plugin
2 - delete the lotro \PluginData folder (the settings are saved in a different way after the patch is applied which is incompatible with the previous formatting, you'll have to recreate them all, sorry

)
3 - rename ApplyPatch.ex_ to ApplyPatch.exe and run it (read below if you do not want to run an unkwown .exe file). You'll need to re-run this script each time you install a new plugin in the \Plugin directory in order to patch also the newly installed plugin).
4 - Launch lotro and Enjoy ! You should now be able to load / unload plugins without anymore "parse" error... The problem with the accentuated characters (in particular with buffbars) should also be fixed.
REMARK:
- If you are uneasy lauching the .exe, you can compile the (very very dirty) source ApplyPatch.cpp or, even better, you can create your own script which take the actions described in ApplyPatch.txt
Hope it helps... have fun !