In case anyone passes this way again and has had trouble finding/understanding information on this as I did, this post on adding Babel lexicons was useful and pointed me in the right direction.
http://forums.modx.com/?action=thread&thread=70846
For the Login plugin, I followed these steps (in 2.2.14 version):
1) Navigated to the core/components/login folder ( via FTP or SSH)
2) Opened the lexicon folder, saw the "en" folder, copied it, renaming it "zh" in my case
3) Went into the Modx manager, clicking on System->Lexicon Mngmt-> then selected "Namespace:login" and "Topic:updateprofile" (as it was the "update profile" screens I was working on). Selected "zh" in Language dropdown, as this had now appeared, due to me creating the folder in step 2
4)Found the appropriate entries I wanted to change in the list e.g. name:login.profile_updated value:Profile Updated. Clicking on Profile Updated allows you to edit and change the message to your new translated phrase.
5)Navigated in Manager to the page/chunk in question where the message was displayed. Found reference to [[%login.profile_updated.......]] and added &language=`zh` to get [[%login.profile_updated?&language=`zh`....
Repeat 4-5 for other messages needing changing.
I presume to process is similar in Modx2.3. I presume also for any other components / plugins, they will also have a lexicon folder in their folder in core/components/
Good luck
