Language code for translation placeholders and translation context?

Question

I'm planning to add a feature to my application where you can switch to the "Translation" locale and then see the names of the translation placeholders in the application instead of the actual translations. Another nice thing are "context descriptions" where you see explanations in plain english what the placeholder actually is for.

My question is: Are there any standardized language/locale codes (e.g. defined by ISO 639-3 or ISO 15897) for these use cases?

If not, I'll probably use a character sequence like qqq or xx_XX.

Answer 1

ISO 639 defines three special codes and a code range for local use:

• [mul]: multiple languages
• [und]: undetermined
• [zxx]: no linguistic content
• [qaa]–[qtz]: reserved for local use (you could e.g. use [qtr] for "translate")