SELECT 
  object_id 
FROM 
  cscart_settings_objects 
  LEFT JOIN cscart_settings_sections ON cscart_settings_objects.section_id = cscart_settings_sections.section_id 
WHERE 
  cscart_settings_objects.name = 'frontend_default_language' 
  AND cscart_settings_sections.name = 'Appearance'

Query time 0.00063

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_settings_objects",
          "access_type": "ref",
          "possible_keys": ["name", "section_id"],
          "key": "name",
          "key_length": "386",
          "used_key_parts": ["name"],
          "ref": ["const"],
          "rows": 1,
          "filtered": 100,
          "index_condition": "cscart_settings_objects.`name` = 'frontend_default_language'"
        }
      },
      {
        "table": {
          "table_name": "cscart_settings_sections",
          "access_type": "eq_ref",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "4",
          "used_key_parts": ["section_id"],
          "ref": ["dev_db.cscart_settings_objects.section_id"],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "cscart_settings_sections.`name` = 'Appearance'"
        }
      }
    ]
  }
}