In support of software internationalization, many programming languages and platforms support a means of obtaining localized resources to be used in the UI that is shown to the user (e.g. Java's java.util.ResourceBundle class). Often, if resources for the user's preferred locale are not available, then there is a fallback mechanism, or locale resolution process, that will attempt to locate the nearest-matching resources from the sets of available resources. For example, if resources for en-US are not available, then commonly the system attempts to find resources for en.
The locale resolution process seems nearly the same for many languages' and platforms' resource bundle solutions. Are they following some standard locale resolution algorithm, or, if not, does such a standard exist?
I'm not aware of a standard per se.
However, the algorithm being used is a trivial consequence of the fact that locales are hierarchical. There is a (notional) root locale with no name. Beneath this are language-only locales (en, fr, etc). Beneath those are national locales (en_GB, en_US, etc). Beneath those are, optionally, variant locales (en_GB_Yorkshire, en_GB_cockney, etc - for realistic examples, look at Norway).
The natural way to find an appropriate resource is to start with the lowest, most specific, locale you can, and walk up the tree until you find something. So, starting with en_US_TX, you step up to en_US, then en, then the root.