SELECT 
  cscart_languages.* 
FROM 
  cscart_languages 
  LEFT JOIN cscart_storefronts_languages AS storefronts_languages ON storefronts_languages.language_id = cscart_languages.lang_id 
WHERE 
  1 
  AND cscart_languages.status = 'A' 
  AND (
    storefronts_languages.storefront_id = 2 
    OR storefronts_languages.storefront_id IS NULL
  )

Query time 0.02510

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_languages",
          "access_type": "system",
          "rows": 1,
          "filtered": 100
        }
      },
      {
        "table": {
          "table_name": "storefronts_languages",
          "access_type": "index",
          "key": "PRIMARY",
          "key_length": "8",
          "used_key_parts": ["storefront_id", "language_id"],
          "rows": 2,
          "filtered": 100,
          "attached_condition": "trigcond(storefronts_languages.storefront_id = 2 or storefronts_languages.storefront_id is null) and trigcond(storefronts_languages.language_id = 1)",
          "using_index": true
        }
      }
    ]
  }
}