Language pack probem Spanish(United States)

Today, we started developing new locale for our existing site that is Hispanic locale for United States, but the main problem which we found is Microsoft does not provide language pack for Hispanic (United States) the solution for this we have chosen (Spanish) Mexico language pack (es-MX)under regional settings of site collection settings of site because the Spanish (Mexico) and Spanish (United States) are almost same

But, before start developing new locale with above approach the resource files should be in the form of .es-MX.resx not .es-US.resx because you have chosen (Spanish)Mexico language pack (es-MX) under regional settings of site collection