1.11 does require resource file names to be lowercase (en_us.lang), but Forge will add mod resources to the resource pack list using the LegacyV2Adapter class if they don't have a pack.mcmeta file or they have one that specifies pack_format as 2 (see FMLClientHandler#addModAsResource).   LegacyV2Adapter will load mixed case (en_US.lang) lang files, but will still require all other file names to be lowercase.   In summary: If you have a pack.mcmeta file with pack_format 3, use