Fix callback url matching for multilingual sites

[anktd]

Issue

This issue/feature request was reported via freshdesk ticket 338848

With WPML active, WC()->api_request_url() returns URL with language prefix:
- German context: https://example.com/de/wc-api/WC_Gateway_Blockonomics/
- French context: https://example.com/wc-api/WC_Gateway_Blockonomics/

The method findMatchingStore() compares this against the callback URL stored on Blockonomics dashboard (which was set up in English, i.e. without /de/)

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Fix

The following 2 patterns cover 99% of Polylang or WPML setups:

Pattern	Example	Usage
/xx/	/de/, /fr/, /es/	Standard ISO 639-1 (most common)
/xx-xx/	/en-us/, /fr-ca/, /pt-br/	Language + region

To tackle both, the fix uses regex verification: /^([^\/]+)\/[a-z]{2}(-[a-z]{2})?\//$1/

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What it won't handle:

• Subdomains (de.example.com)
• Different TLDs (example.de)
• 3-letter language codes (/chi/)