You should think about how the localization process will take place. I assume you are not a native speaker in all languages you want to use for your application.
There are several approachs on how to address this: For example, there are companies that specialize in localization, meaning you give them an excel sheet, or an xml file.
You should also think about, where do you want to have all these localizations. Do you only want them in your ASP.net application, meaning in only one place? Then the resource file will be your way to go, because they are easy to handle and easy to send to localization studios.
But if you want to use the localizations in more than one place, you need to store them in a web service or in a database. Keep in mind that using localizations across multiple plattforms (e.g. web site, administrative tools) will force you to write import/export functionality for the used tables. (Because you won't give the localization company access to your database)