Make Translation::offsetExists reflect real translation keys

[javihgil]

Problem

Translation::offsetExists() currently always returns true.

Current behavior

Consumers using isset($translation['locale']) cannot tell whether a translation exists for that locale.

Expected behavior

offsetExists() should return true only when the requested key is actually present in the translation payload.

Notes

This breaks the usual ArrayAccess contract and can lead to incorrect fallback logic in consumers that rely on isset().