Use ASP.NET localization with .resx resource files: create Page.aspx.resx (English) and Page.aspx.fr.resx (French), set UICulture="auto" Culture="auto" in the page directive, store the selected language from the dropdown in Session (e.g., "en" or "fr"), set Thread.CurrentThread.CurrentUICulture and CurrentCulture in Global.asax (Application_BeginRequest), then reload the page so the correct language content loads automatically from the corresponding resource file.
Ex: https://www.aspsnippets.com/questions/876678/How-fnf-to-show-aspx-project-in-multiple-language/