I created an ebook with Sigil, then I converted it to mobi using Calibre. While I was at it, I edited (Calibre's) metadata and set Italian as language. A friend of mine warned me that hyphenation was English and not Italian: indeed I reopened the file with Sigil and found that in content.opf the language was en, not it.
Is this a known issue / a feature?
Calibre's metadata.opf is only used for Calibre purposes. If you change metadata entries with Calibre, the epub isn't immediately updated. IMHO, this is a major bug, but Kovid Goyal apparently considers it a feature.
If you want to apply the changes to the book, you'll have to select Save to Disk and save the book to a new folder. Alternatively, install the Modify Epub Calibre plugin and select Metadata > Update Metadata.