Extension Management Language Bugs

Noticed some missing language vars, and improper usages of language vars too, in the extension manager

Some keys use EXTENSION_ when it should be EXTENSIONS_ because that is what the exception_prefix is defined as.

Some unintended re-usages of keys are causing that HTML table for how to Install, Update and Remove extensions to be used, HTML and all, in title attributes for mouse hovering over the Update or Remove links.