SELECT 
  cscart_vendor_plan_descriptions.*, 
  cscart_vendor_plans.* 
FROM 
  cscart_vendor_plans 
  LEFT JOIN cscart_vendor_plan_descriptions ON cscart_vendor_plan_descriptions.plan_id = cscart_vendor_plans.plan_id 
  AND cscart_vendor_plan_descriptions.lang_code = 'ja' 
WHERE 
  (
    status IN('A')
  ) 
  AND (
    FIND_IN_SET(1, storefronts) 
    OR storefronts IS NULL 
    OR storefronts = ''
  ) 
ORDER BY 
  position asc

Query time 0.00089

JSON explain

{
  "query_block": {
    "select_id": 1,
    "read_sorted_file": {
      "filesort": {
        "sort_key": "cscart_vendor_plans.position",
        "table": {
          "table_name": "cscart_vendor_plans",
          "access_type": "ALL",
          "rows": 5,
          "filtered": 100,
          "attached_condition": "cscart_vendor_plans.`status` = 'A' and (find_in_set(1,cscart_vendor_plans.storefronts) or cscart_vendor_plans.storefronts is null or cscart_vendor_plans.storefronts = '')"
        }
      }
    },
    "table": {
      "table_name": "cscart_vendor_plan_descriptions",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "10",
      "used_key_parts": ["plan_id", "lang_code"],
      "ref": ["smscrm_edev.cscart_vendor_plans.plan_id", "const"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "trigcond(cscart_vendor_plan_descriptions.lang_code = 'ja')"
    }
  }
}