<p>What is the best way to support multiple languages for the interface in an ASP.NET MVC application? I've seen people use resource files for other applications. Is this still the best way?</p>

Yes resources are still the best way to support multiple languages in the .NET environment.  Because they are easy to reference and even easier to add new languages.
```
```
Site.resx
Site.en.resx
Site.en-US.resx
Site.fr.resx
etc...
```
```
So you are right still use the resource files.