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 0.00102

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "0.70"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_languages",
          "access_type": "ALL",
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 1,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "0.25",
            "eval_cost": "0.10",
            "prefix_cost": "0.35",
            "data_read_per_join": "216"
          },
          "used_columns": [
            "lang_id",
            "lang_code",
            "name",
            "status",
            "country_code"
          ]
        }
      },
      {
        "table": {
          "table_name": "storefronts_languages",
          "access_type": "index",
          "key": "idx_storefront_id",
          "used_key_parts": [
            "storefront_id"
          ],
          "key_length": "4",
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 1,
          "filtered": "100.00",
          "using_index": true,
          "using_join_buffer": "hash join",
          "cost_info": {
            "read_cost": "0.25",
            "eval_cost": "0.10",
            "prefix_cost": "0.70",
            "data_read_per_join": "16"
          },
          "used_columns": [
            "storefront_id",
            "language_id"
          ],
          "attached_condition": "(<if>(found_match(storefronts_languages), ((`aerokidsshop`.`storefronts_languages`.`storefront_id` = 1) or (`aerokidsshop`.`storefronts_languages`.`storefront_id` is null)), true) and <if>(is_not_null_compl(storefronts_languages), (`aerokidsshop`.`storefronts_languages`.`language_id` = `aerokidsshop`.`cscart_languages`.`lang_id`), true))"
        }
      }
    ]
  }
}