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 (
    storefronts_languages.storefront_id = 1 
    OR storefronts_languages.storefront_id IS NULL
  )

Query time 0.00055

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "storefronts_languages",
      "access_type": "system",
      "possible_keys": ["PRIMARY", "idx_storefront_id"],
      "rows": 0,
      "filtered": 0,
      "const_row_not_found": true
    },
    "table": {
      "table_name": "cscart_languages",
      "access_type": "ALL",
      "rows": 3,
      "filtered": 100
    }
  }
}