Like: Do we call it File or Document. Especially important if you make an interface that will be adopted for several languages.
It is called Copy. See 'Copy' on Wikipedia:
Copy refers to written material, in contrast to photographs or other elements of layout, in a large number of contexts, including magazines, advertising, and book publishing. In advertising, web marketing and similar fields, copy refers to the output of copywriters, who are employed to write material which encourages consumers to buy goods or services. In publishing more generally, the term copy refers to the text in books, magazines, and newspapers. In books, it means the text as written by the author, which the copy editor then prepares for typesetting and printing. In newspapers and magazines, "body copy", the main article or text that writers are responsible for, is contrasted with "display copy," accompanying material such as headlines and captions, which are usually written by copy editors or sub-editors.
Are you thinking of a "[Localization] Resource File"? See MSDN definition here.
Resource files are typically any non-executable data file used by your application, such as image, audio, and video files. A resource file can also have specific meanings in certain contexts. For example, in the context of application localization, resource files refer to .resx files, which you can deploy in localized satellite assemblies.
And technical details about how to do this in .NET here