Sandbox

SELECT 
  COUNT(*) 
FROM 
  cscart_product_feature_variants 
  LEFT JOIN cscart_product_feature_variant_descriptions ON cscart_product_feature_variant_descriptions.variant_id = cscart_product_feature_variants.variant_id 
  AND cscart_product_feature_variant_descriptions.lang_code = 'ja' 
WHERE 
  1 
  AND cscart_product_feature_variants.feature_id IN (18)

Query time 0.00004

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_feature_variants",
      "access_type": "ALL",
      "possible_keys": ["feature_id"],
      "rows": 158,
      "filtered": 33.54430389,
      "attached_condition": "cscart_product_feature_variants.feature_id = 18"
    }
  }
}

Result

COUNT(*)
55