Localisation is part of content negotiation, and language preferences are specified using the Accept-Language
header (RFC).
The RFC is quite strict about what the service is allowed to return here — for instance, if no header is sent, then you may choose any language, but if a header is included, then you may only return one of the requested languages, and are not allowed to fall back to a different language.
In other words, if a resource is only available in English, then a resource requested with Accept-Language: de
should return 406 Unacceptable
rather than default to presenting the English version.
Other alternatives that don't use the specification but do allow for fallback options are discussed in a couple of other questions (8204816, 7892502).