7 Billion people in the world do not speak english, and although a user may be able to muddle through your software or website if english is their second language, make life easier for them, and provide your website or software in your user's language.
Improve your website's SEO position by hitting keywords in foreign markets. Non-english speaking users always search Google in their first language before english. Compete in non-english markets, and reach another 7 Billion potential customers.
Instead of re-inventing the wheel, if you are going to provide your .NET application in multiple languages, use .RESX, the native Microsoft format for resource files. They are compiled along with your DLLs, for fast and efficient access.