@zephyrot,
As a heavy user of making MSWord work with Calibre, I have to say that 'encoding' is one of the few things which has never given me a problem. So I wonder if we might go back to square one.
I have attached a tiny epub (created from MSWord to Webpage-filtered) with some Spanish chars in it.
The first image below is what it looks like in the Calibre viewer. The second image is a screencap from my Sony PRS505. As you can see the Spanish chars display correctly.
I'm afraid I don't have any experience with iPod or Stanza but perhaps you could do the following simple test as a first step:
- View the attached epub in your Calibre viewer and confirm that it looks OK on your PC.
- Transfer it to your reader in your normal way. Is it still OK?
If 1. is OK and 2. isn't then the problem is at the iPod/Stanza end not the Calibre conversion. If both are OK then we need to take a closer look at your source file and the conversion settings.