Do a search for 'E104' on the forum - you will see there are a number of posts on this subject.
Generally, as you say - form E104 is not one of the forms listed by the HO guidance as examples of what can be used to prove CSI coverage. I've heard of people having their PR rejected when they sent form E104 as proof of CSI, but I've also heard of people who have had their PR accepted when sending form E104. So it's hard to give a definite answer.
I know of one person who sent a certified translation of form E104 + a free-form letter issued by their government confirming past EHIC coverage, and the HO accepted this and issued the PR.
Any document not in English must be translated by a certified translator - see
www.iti.org.uk for a list of translators. I had a one-page document translated online for my PR application - cost me £50.