SELECT 
  lang.value 
FROM 
  cscart_language_values lang 
WHERE 
  lang.lang_code = 'ru' 
  AND lang.name = 'block_menu'

Query time 0.00058

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "lang",
      "access_type": "const",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "392",
      "used_key_parts": ["lang_code", "name"],
      "ref": ["const", "const"],
      "rows": 1,
      "filtered": 100
    }
  }
}