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 = 'fr' 
WHERE 
  cscart_product_feature_variants.variant_id = 240

Query time 0.00024

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_product_feature_variants const PRIMARY PRIMARY 3 const 1
1 SIMPLE cscart_product_feature_variant_descriptions const PRIMARY PRIMARY 9 const,const 1

Result

variant_id feature_id url position variant description page_title meta_keywords meta_description lang_code custom_header
240 30 40 VELUX fr