language of error messages and other strings

[nichtich]

should be localized by the HTTP accept-language header or sent in a default language.

[JonathanRowell]

PAIA is an interface and not a GUI. All messages, as in HTTP, should be in English and the client should provide a translation.

[nichtich]

We should have a look at common error types (#33) before further decision.

[nichtich]

Possibly language dependent fields in PAIA are limited to:

• doc.about textual description of the document
• doc.error error message, for instance if a request was rejected
• patron.name full name of the patron
• patron.address freeform address of the patron
• doc.storage Requested pickup location (but there is also doc.storageid)
• fee.about textual information about the fee
• fee.feetype textual description of the type of fee (may be renamed, see Modify fee.feetype and fee.feeid #44)
• error_description Human-readable error description (in request errors)

[nichtich]

After thinking more about this I decided to refer to good old Content-Negotiation (PAIA 1.0.8): 143f118

@JonathanRowell Translations provided by clients are only possible based on identifiers such as the error field of request errors. We don't have such identifiers for document errors, so I opened #45