SELECT 
  * 
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 = 'en' 
WHERE 
  cscart_product_feature_variants.variant_id = 3709

Query time 0.00040

JSON explain

{
  "query_block": {
    "select_id": 1,
    "const_condition": "1",
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_feature_variants",
          "access_type": "const",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["variant_id"],
          "ref": ["const"],
          "rows": 1,
          "filtered": 100
        }
      },
      {
        "table": {
          "table_name": "cscart_product_feature_variant_descriptions",
          "access_type": "const",
          "possible_keys": ["PRIMARY"],
          "key": "PRIMARY",
          "key_length": "9",
          "used_key_parts": ["variant_id", "lang_code"],
          "ref": ["const", "const"],
          "rows": 1,
          "filtered": 100
        }
      }
    ]
  }
}