Wikipedia is an international project, and contains many individual Wikipeidas in many Languages. The largest of these by far is in English. As such, many users whose first language in not English regulary visit, or even edit, on English Wikipedia. In addition, many English speakers also visit/edit on the Wikipeidas in other languages.
English is the only language in which I am fluent,and I am not able to understand any other languese. However, I often find the contributions on the other Wikipedias to be very useful, especially in finding sources or photos. WHen I crete or expand an article in English WP, I often go to some of the interwikis to see if they have articles, and what those articles are like.
One tool which is of great help in using interwikis in the login. This enables one to be a member of other Wikipedia and Wikimedia projects, and to use the same log-in name on all of them.
However there are other ways to impove one's experiance in using the other Wikipeidas.
My favorits one is the Internationalization feature, which is found on the main My preferences page. This enables one to set a specific language as one's interface language. As English is my primary language, I set my preference to English. This enables most of the headings and sidebars to show in English.