Safari Extensions are in:
per-user: ~/Library/Safari/Extensions
General "Netscape API" browser plug-ins are in:
per user:
~/Library/Internet\ Plug-ins/
and
system-wide:
/Library/Internet\ Plug-ins/
Honestly, the problem with localized strings not found seems like corruption or a missing file within the Safari.app bundle. Like maybe a missing or corrupt copy of this file:
/Applications/Safari.app/Contents/Resources/English.lproj/localizedStrings.js