Are those past releases all stable releases?

Yes, even if source releases with a version <= 0.10 exist because of the book about Swing (each version 0.1, 0.2 match an Agile scenario explained in the book). As stable, I mean it's not buggy (or contains bugs I wasn't aware of).

If so, how were these stable releases done in the past?

I just tagged Sweet Home 3D CVS repository with a tag matching the version.

Any voting system involved or any decision strategy used?

No vote is required since I'm the only developer and committer until now. SourceForge.net suggests to follow the maxim "release early, release often", but this is quite vague even if a recent article gave more information about this strategy. My current strategy would be to release a new versions 3 or 4 times a year, and trying to group changes in the user interface as much as possible in fewer sub versions, because each time I add a new word in the user interface, I have to send 14 emails (and sometimes wait for some answers during weeks) to update the 16 languages included in Sweet Home 3D. I also don't want to release too often because creating, checking, rechecking, uploading released files, updating history, publishing a new blog article is a tedious task that takes often more than 4 hours; and once you released a new version, other web sites (like app stores containing links pointing to released files) report the new version. I'm not sure these web sites as well as Sweet Home 3D users would appreciate a new version every day.

Well, I use Evrsoft First Page 2006 as the HTML editor. Is it fine?

Any editor will be ok, as long as you don't break the current presentation. You should be able to use utf-8 for Chinese too, but don't worry about this and choose the encoding you want. I'll transcode the text to utf-8 once done, whatever the encoding you used. After managing all Sweet Home 3D translations, I got used to encodings pretty well. [;)]
I you wonder why utf-8, it's because it's simpler to have only one encoding for the whole web site, and only utf-8 contains the characters of the tooltip of each country associated to the flags displayed on the web site.

Could you tell me how to deliver the modified file once I'm done, by email?

Send me by email the translated HTML file once done. It will be perfect. sweethome3d.com web site pages aren't in CVS yet, but I made some small changes recently to clean the code and will publish it sooner or later.