What is wrong with my tree, the old leaves from dark green turned into this:

What is it lacking? And how do I fix it?

Let me remind you that I'm still in need of help
Yellowing leaves can mean:
Potassium deficiency, copper deficiency, molybdenum deficiency.
WHAT DO YOU THINK IT IS IN MY CASE? HOW DO I FIX IT?
I use a fertilizer for citrus plants that contains all those elements.
Or the leaves are just getting old? Because they already were on the tree when I obtained it. They don't fall off, they just get yellow and stay on the tree. And should I remove them or not?

These leaves don't look like they have any deficiency to me. They just look like old leaves whose time has come. It is common for citrus trees to drop more old leaves at this time of year. Are they the oldest leaves on the tree? How long have you had this tree? Has it had a big environmental change lately? It could also be winter leaf drop. I have noticed different citrus varieties will drop their old leaves at different stages of yellowness. My Meyer lemon will stubbornly hold its old leaves until they are completely yellow before it drops them while some other varieties will drop their old leaves after only a slight change in color.

Well I bought the tree in the middle of January, so the leaves are definitely older than that.
Winter leaf drop? It's spring here ! The days are getting longer, warmer and etc.

Winter leaf drop is at is worst precisely when the days get sunnier and it starts to warm up in the spring. It is caused by an imbalance in the temperature between the leaves and the roots. When the sun is shining on the leaves they heat up to a temperature far above the root and air temperature and they transpire more water than they receive from the cold roots. In response, the tree will drop leaves to reduce transpiration.

The usual solution is either to heat the roots or partially shade the leaves.

Leaves can be partially shaded using a sheet or shade cloth or by moving the tree out of direct sunlight. I suspect air movement from a fan on the leaves will also help but I am not sure.

Roots can be heated using a heating mat or heating cable or by allowing the sun to shine on the container.

Search for winter leaf drop or WLD on this forum. There are many posts on the subject.