I've installed a MediaWiki site. I think default MediaWiki supports only one language which is configured during installation.
Is there a way in MediaWiki to support two or more languages like wikipedia.org? Available languages for a page should be listed on left side like Wikipedia, and when a user clicks a language, the version of the page in the selected language can be seen.
What is the conventional way to support multilingualism?
There are two different ways to make a multilingual wiki setup.
You can have a family of wikis each of which supports a different language. This is how the Wikipedias work (en.wikipedia.org, ru.wikipedia.org, es.wikipedia.org, etc). This is probably what you are aiming for. Pay special attention to the section on configuring interwiki links to get the links on the left hand side of the page.
You can create one wiki that supports multiple languages. This is how meta.wikimedia.org works. For that, you want to use the Translate extension.