zxx
zxx is the ISO 639-3 code for No linguistic content. It is a special code used to identify content that does not contain any linguistic material, such as text in a natural or constructed language or spoken language.
Purpose and usage: The code is used in metadata and language tagging to indicate that a resource
Relationship to other codes: zxx is one of several special codes in the ISO 639 family. It
Examples: materials that would be tagged zxx include diagrams, mathematical notations, charts, graphs, logos, musical notation