As far as I know, K2 in its current version does not support Item Associations, similar to what you get in Joomla's core articles, when you enable the feature in the language filter plugin.
One possible workaround to create associations among the k2 items is by creating menu items for those k2 that you want to be associated. You will actually associate the menu items. This could work for a certain number of k2 items, it won't be an option if you have hundreds of them.
Otherwise, a 3rd party translation extension like falang could be the solution.