SELECT 
  b.*, 
  d.*, 
  c.* 
FROM 
  cscart_bm_blocks as b 
  LEFT JOIN cscart_bm_blocks_descriptions as d ON b.block_id = d.block_id 
  LEFT JOIN cscart_bm_blocks_content as c ON b.block_id = c.block_id 
  AND d.lang_code = c.lang_code 
WHERE 
  b.block_id = 138 
  AND d.lang_code = 'en' 
ORDER BY 
  snapping_id DESC 
LIMIT 
  1

Query time 0.00049

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.005726018,
    "const_condition": "1",
    "filesort": {
      "sort_key": "c.snapping_id desc",
      "temporary_table": {
        "nested_loop": [
          {
            "table": {
              "table_name": "b",
              "access_type": "const",
              "possible_keys": ["PRIMARY"],
              "key": "PRIMARY",
              "key_length": "4",
              "used_key_parts": ["block_id"],
              "ref": ["const"],
              "rows": 1,
              "filtered": 100
            }
          },
          {
            "table": {
              "table_name": "d",
              "access_type": "const",
              "possible_keys": ["PRIMARY"],
              "key": "PRIMARY",
              "key_length": "10",
              "used_key_parts": ["block_id", "lang_code"],
              "ref": ["const", "const"],
              "rows": 1,
              "filtered": 100
            }
          },
          {
            "table": {
              "table_name": "c",
              "access_type": "ref",
              "possible_keys": ["PRIMARY"],
              "key": "PRIMARY",
              "key_length": "4",
              "used_key_parts": ["block_id"],
              "ref": ["const"],
              "loops": 1,
              "rows": 4,
              "cost": 0.005726018,
              "filtered": 100,
              "attached_condition": "trigcond(c.lang_code = 'en')"
            }
          }
        ]
      }
    }
  }
}