I've created a font family, but Font Book refuses to include it in the English language set, despite my best efforts.
The font has every glyph in the OpenType "Std" set, plus several others.
I've checked various boxes for Latin 1 and Macintosh Character Codepages; plus Unicode ranges for Basic Latin, additional Latin, etc, etc.
I've compared it to several other fonts that are in the English set, and I can't see anything that they have that my fonts don't. (In fact, many of them seem to have much less!)
I've created other fonts that are in the English set, but I've no idea what the difference is.
Given that macOS relies on these Language sets, in order to hide the thousands of unnecessary fonts that are permanently installed in the OS, there ought to be some guidance on how to do this.