How to install a 102..105 windows keyboard (USB)
in a iMac (OSX Yosemite 10.10.3 in this case)
in foreign languages layouts...
I want PT-PT in this case...
but I extend my question to other languages
(Spanish, French, Italian, etc.)
Usually, you only have to plug in your keyboard. OSX will then recognized it and ask you to type on some keys so as to recognize the layout. This will not always work.
I personally have a traditional PC keyboard on my mac. I use the layout switcher in menu bar to quickly change between my built-in mac keyboard and the PC one. You can find the various layouts in System Settings --> Keyboard --> Layouts (the 4th tab).
EDIT :
If none of the given layouts is satisfying you better follow this question to create your custom layout.