SELECT 
  * 
FROM 
  cscart_bm_locations as l 
  LEFT JOIN cscart_bm_locations_descriptions as d ON d.location_id = l.location_id 
  AND d.lang_code = 'fr' 
WHERE 
  l.layout_id = 2 
  AND l.is_default = 1 
ORDER BY 
  l.is_default desc

Query time 0.00026

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "l",
      "access_type": "ALL",
      "rows": 30,
      "filtered": 100,
      "attached_condition": "l.layout_id = 2 and l.is_default = 1"
    },
    "table": {
      "table_name": "d",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "10",
      "used_key_parts": ["location_id", "lang_code"],
      "ref": ["chevnudb1.l.location_id", "const"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "trigcond(d.location_id = l.location_id and d.lang_code = 'fr')"
    }
  }
}