Fix partner/connect endpoint error when used with incompatible WordPress locale.

[budzanowski]

The https://api.pinterest.com/v3/catalogs/partner/connect/ endpoint supports only the following locale

it,es-419,ru-RU,hu-HU,hr-HR,sv-SE,de,nb-NO,th-TH,sk-SK,ro-RO,es-MX,uk-UA,nl,en-AU,he-IL,tr,es-ES,pl-PL,cs-CZ,en-CA,fi-FI,pt-PT,el-GR,ja,ar-SA,fr-CA,en-GB,es-AR,da-DK,zh-CN,en-US,vi-VN,id-ID,bg-BG,zh-TW,en-IN,tl-PH,ko-KR,af-ZA,ms-MY,bn-IN,te-IN,fr,hi-IN,pt-BR

the task is to implement mapping of WordPress native locales that are required in a different format by Pinterest. For example:
nl-NL should be mapped to nl

Throw an error when no mapping is possible.

The following issue will track a set of items required to finalize the task.

Code tasks 👨🏻‍💻

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1. Add WordPress locale mapping to Pinterest API locale. #683
   type: enhancement +1
   
2. Add unit tests for PR that will implement #683 #684
   type: enhancement +1
   

Documentation 📄

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1. Update documentation with locale mapping implementation details. #685
   type: enhancement +1