The file extension LNG is used by various applications and games for language, ie language files. These are usually text files that contain a list of all messages, menu items etc. In the standard language (mostly English) with a correspondence in another language, eg Welcome = welcome. Alternatively, numbers are used, among other things.
LNG files can only be used by the associated program because it is not a uniform format. Lng files can not usually be loaded directly, but must be stored in a specific folder within the program folder, eg 'language'.
For example, LNG files are used by Microsoft Office, Acrobat, Opera, XnView, Winamp, or TotalCommander.