If you are looking for a way which determine how many ( and what ) languages you localize for your application. There is no solution.
You have to write a parser which look in a series of sub directories in your application ( or given ) path. the read and store the name of Resx files into a list.
Finally you have to split the name of Resx file with Dot (split('.')) and seperate the language part of the Resx files like
string[] myString = new string[MyResxList.lenght];
for (int i=0; i<=MyResxList.lenght;i++)
myString[i] = MyResxList[i].toString().split('.')[3];
note that above code is a snippet and I wrote it here so you have to debugit if it's necessary
then you should remove the duplicates and return the List