SELECT 
  feature_id, 
  variant_id, 
  value, 
  value_int 
FROM 
  cscart_product_features_values 
WHERE 
  product_id = 651 
  AND lang_code = 'de'

Query time 0.00041

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_product_features_values ref lang_code,product_id,product_id_idx,product_feature_idx product_id 3 const 26 Using where

Result

feature_id variant_id value value_int
32 0 2500.000
33 0 1250.000
127 5895
128 5899
129 5912
30 5893
178 6326