Login.html localization and adjusting of wording

First, the <p> tags are not supported by the localization module; words to be translated must be wrapped inside <span> tags.

Second, to make new users less confusing, I changed a little bit of the wording for the 3 use cases: new users, existing users logged in locally and existing users not logged in locally. This part is debatable and shall be reviewed by native English speaker.