I just added Sieben Jahre in Tibet. Mein Leben am Hofe des Dalai Lama (Work) – BookBrainz and there are various ways of separating the title and the subtitle. A full stop, a dash, colon and semicolon.
@indy133 is there a library standard such as the colon in English?
Unfortunalely not. Wiki uses a full stop in most cases, but also a hyphen and a colon.
And if you have a look at the “Katalog der deutschen Nationalbibliothek” its looks like this:
Obviously no rules. For works I am using the colon. For editions “as on” and if there only are different fonts without seperator I also use the colon.
But I really don’t think this to be very important and other users will use other seperators for sure.
That’s easy as no one can make a mistake. Thanks.
FYI in MB we always use colons to separate subtitles, unless there is evidence of specific intent to use something else.
Not sure if this translates well to BB, but if it can we probably should keep them both aligned.
Even if there are existing seperators (eg. brackets or hyphen) printed on the release?
Does that really work for the users?
I certainly adhere to that rule for English language titles with subtitles, but I’m flexible depending on respective language title formatting conventions.
Is there a decree that the rule applies to all languages?
I there is an indication of intent to use different dividers, we would keep them.
For instance, Japanese releases are specific about the characters used (often very stylized), and we don’t usually standardize them.
But very often a release has a colon on the disc, a hyphen on the spine, and so on… then we apply the ‘standard’. And if course when there’s just a picture w. no divider.
We often go a step further and assume that 99% of artists don’t have any care about dividers used, and replace printed ones too. I’ve noticed it change a bit with digital releases, where I’m seeing the store-used divider retained much more - presumably because that name becomes canonical/what people search for/want to use, for that release.
I’ve always wanted a subtitle field 
That sounds like a very good idea.
P.S. sometimes a reply doesn’t need 20 characters!
In general we use the colon for all languages. For some languages (IIRC Japanese) that looks very foreign so we go with whatever makes sense for the language, but if there’s no clear way the language does it, I’d certainly go with colon too, just because it’s simpler. Agreed to use what is printed if something is consistently printed though for edition titles, we do that for releases even in English 
