Placards and Markings
Where can I find regulations pertaining to interior placard language requirements?
For small aircraft there are regulations concerning certain required placards, such as FAR 23.791 - Passenger Information Signs and FAR 23.1557 - Miscellaneous markings and placards. These are certification standards that the FAA requires manufacturers to adhere to.
Manufacturers then publish a list of required placards and the verbiage to use.
Interior and exterior placard requirements can be found in Chapter 11 of the aircraft maintenance manual or illustrated parts catalog.
They usually identify the placards required by the FAA. These are shown with an asterisk next to them or a flag marker saying FAA required.
If you meant where can you find a regulation that identifies which language to use, I am not aware of one. Because they are U.S. regulations, it is implied that English must be used. If it is to be dual language, one language must be in English in order to meet the requirements.
Each country's authority may also require the placards to be in their native language. As a DAR we would find this information in the special import requirements for that country. Click this link to find the requirements for a certain country. AC 21-2, Appendix 2