Took me a while to find this but here is my solution:
link_to 'English', url_for( :locale => 'en' )
link_to 'Deutch', url_for( :locale => 'de' )
From the docs here: http://api.rubyonrails.org/classes/ActionController/Base.html#M000649
When generating a new URL, missing
values may be filled in from the
current request‘s parameters. For
example, url_for :action =>
‘some_action‘ will retain the current
controller, as expected. This behavior
extends to other parameters, including
:controller, :id, and any other
parameters that are placed into a
Route‘s path.
So using url_for will default to the current request’s parameters, just change the one’s you want in your code. In this case all I changed was :locale, so everything else stays the same.
Note this also works for “hidden” :parameters. So if you have:
map.my_map ':locale/my_map', :controller => 'home', :action => 'my_map'
using the above url_for in the page /en/my_map will not have ‘home’ in the url (ie /en/home/my_map). Bonus.