In my asp.net mvc project all text is in resource files, and in view for text values roots to the resource file. I have english and german text. Current culture is read from cookies and rooted from global.asax.
Will this work correctly with search robots or is it better to have an additional parameter value like /en/ in my url?
For search robots like google this will not be any problem.
Google is most professional search engine and has the possibility to see the difference between languages.
Anyways it will link most of the time to your index.
The question is really if search engines will use/submit a cookie, and with that be able to get to the pages in other languages. See is-localization-using-cookies-search-engine-compatible.
Make sure you also have a non cookie based way to get to those pages.