first, is it your saddle?
Because if it is yours then you might as well go for grease. You can even do soap first to get the dirt out, and then use the grease to nourish the leather and make it shine.
You can use grease by itself, but I prefer to clean the saddle first just so the nasty things don't stay on my saddle, but that's just me.
Then, I see your are buying from decathlon, well known store in france, where are you located? If you have access to decathlon, I would suggest a grease that I have been using for 10 years now, and it is awesome!
My saddle and half chaps are about 5 years old, and my bridle is almost 10years old, and all in excellent conditions. Thanks to my grease EFFAX BAUM CUI- GO SPORT
This is another french store, but if you google effax lederbalsam and find online stores that carry it