In this context you need to use $remote->user, not $u->user. User methods can't be interpolated in strings, so it would look like this:

"/user?journal=" . $remote->user . "'>Subscribe...

Also, to add your new translation string, you will want to define it in /community/manage.bml.text. Name it .commlist.subscribe or whatever and use $ML{} to load it as above.