Keyboard no longer writes certain letters?

For a few days my laptop computer has stopped writing certain letters.

So far I've identified "null", "comma", "o", "k", "comma". And I can only write the comma, o, and k, because I'm copying it from other texts using CtrlC…

Sometimes it's gone again, but most of the time the problem is there.

Yesterday I set up my laptop again (formatted, reinstalled, etc.) and unfortunately the problem survived. Is it considered a hardware / mechanical problem?

If it persists even after re-installing it, it is probably a mechanical problem. Can also be crumbles that are stuck under the button.

Of course, if the crumbs or dust are under a key, why should they move away? Try it carefully with compressed air and a brush.
Alternatively, as you already write, an external person. Or, depending on the notebook model, a new internal one.

Actually, I wanted to write "Check which keyboard layout is active. Did you accidentally switch to the US keyboard or something?"

But the affected keys are very close together, so it's probably dirt (something may have gotten in). Can you pull and clean the buttons?