No need to reinstall - the problem is obviously elsewhere. For avast! to work correctly in other language versions, the corresponding language has to be set in the operating system as default.
nirvana74v, what operating system do you have? In WinXP, for example, the setting is called "Language for non-Unicode programs" (in Regional and Language Options, "Advanced" page). Unless it's set to Russian, you won't see the Russian dialogs in avast! correctly.