Support localized problem detail messages

[jpraet]

The problem RFC 9457 allows for localized problem title and detail.
Belgif guidelines state the problem title should be in English (why?).

The requested language can be specified in an Accept-Language HTTP request header (see https://www.belgif.be/specification/rest/api-guide/#i18n-negotiation).

The rest-problem-java library could support localized problem (and issue) detail messages.

[jpraet]

@pvdbosch do you know why the Belgif guidelines require the problem title to be in English, while the RFC 9457 does allow for it to be localized?

If that restriction is kept, maybe it should be added to the "This guide applies following additional restrictions on the Problem Details structure:" section.

[pvdbosch]

The reason was that the problem title is meant for technical support people. They don't necessarily use the same language as the end user, and it's probably more beneficial to have a single recognizable message then.

The schema provides a bit more explanation: Written in English and readable for engineers (usually not suited for non technical stakeholders and not localized).