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 = 'de' 
  AND (
    (
      c.object_id = 43212 
      AND c.object_type like 'categories'
    ) 
    OR (
      c.object_id = 0 
      AND c.object_type like ''
    )
  ) 
ORDER BY 
  snapping_id DESC 
LIMIT 
  1

Query time 0.00031

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.005726018,
    "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": "c.block_id <=> 138 and c.lang_code = 'de' and (c.object_id = 43212 and c.object_type like 'categories' or c.object_id = 0 and c.object_type like '')"
        }
      }
    ]
  }
}