@DNSB
Thanks for your help. I did two mistakes while renaming four files which is indeed not very brilliant... Now it's OK.
But, if each ebook dealer requested like Kobo to use special naming conventions for fonts, what a mess! As a new Kobo user, I am still very surprised by these - ... - requirements.
Chacun son goût, of course. I like reading with Linux Libertine and dislike to read with over-weighted fonts. I understand very well that others make other choices.
When I was speaking about "official" source for Linux Libertine, I only thought about their own site, not about Kobo. I should have precised it. I meant to say that it was not any kind of custom-built subset font, western languages only for example, or something like that. It was the REAL thing, straight from the horse's mouth.
Thanks again.