Tanks @zoltron! That’s exactly what I ended up doing. I found some matte English stickers and they work really well. The only thing is, as you said, you lose a bit of the backlighting, but I don’t usually use it anyway, and I only replaced the symbol keys. Problem solved ;-)

You may find keyboard stickers on online shops like ebay. In Spanish or Portuguese may be hard to find but english is fairly easy. 13 years ago i saved 300 Euros by buying a laptop in the UK. I bought stickers in Italian and ended up cutting the stickers to get the symbols i needed for Portuguese. The stickers are not super-thin but they can be cut using a nailclipper.

In my case i used transparent stickers with red symbols. I wanted to see the English keyboard and the Portuguese symbols as I sometimes alternate.

The drawbacks:

• If the keyboard has backlighting, the light won't go through the sticker

• After a while you may notice that the stickers will start to "unstick" mostly because of heat from the laptop and fingers 

As a quickfix you may also try to print the symbols on paper, cut them and place them on the keys with some tape. 

I suppose that buying spare keys would be more expensive (2+ eur/key on the slimbook shop) but there may be a way for your company to pay for it :-) .

Good luck!