Use 'locale' method instead of '@locale' variable

Use a private 'locale' method instead of the '@locale' instance variable.
Remove unused 'generic_page'
Avoid using '@page' variable with known value
Use 'page' method instead of '@page' variable

Use 'page' private method instead of '@page' instance variable.
Comment out unused 'render_registration_page'

Comment out unused anymore private meths 'render_registration_page'.
Simpler code
Set locale in a "before filter", commit 1/2
Set locale in a "before filter", commit 2/2:

normalise indentation.
Use URLs with locale for downloading
Move a "before filter" to 'filters' file
Use more general language selection links:

do not use the value of 'page', use 'request.path_info' directly.