SELECT 
  * 
FROM 
  cscart_cp_pr_for_seo 
WHERE 
  thread_id = 20374 
  AND lang_code = 'uk'

Query time 0.00029

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_cp_pr_for_seo",
          "access_type": "const",
          "possible_keys": ["thread_id_lang", "thread_id"],
          "key": "thread_id_lang",
          "key_length": "10",
          "used_key_parts": ["thread_id", "lang_code"],
          "ref": ["const", "const"],
          "rows": 1,
          "filtered": 100
        }
      }
    ]
  }
}