Using translation module, create 2 nodes, the first one is the English one (node 1) and the second one is e.g. the Spanish translation (node/2).
So the proper path to access those nodes are: en/node/1 and es/node/2. However, I can also go to es/node/1 and en/node/2, which will display the nodes as it is but the interface language will be different. This doesn't really make sense, because node/1 is an English content but you can view it with Spanish interface (the title, links, descriptions etc are all in Spanish).
There should be a setting to turn off this unneeded pages. It is also bad for SEO because search engines will regard es/node/1 and en/node/1 as duplicate content.
(this is also discussed a little bit here: http://drupal.org/node/67767 )