For notification email subjects, we should use quotes where needed to prevent ambiguity. The normal messages deal with this by using bolding, but that's unavailable in email subjects.
For example: Roan thanked you for your comment in Foo on Bar on Baz is ambiguous. In the interface it looks like Roan thanked you for your comment in Foo on Bar on Baz, which resolves the ambiguity. For the email message, we could use Roan thanked you for your comment in "Foo on Bar" on Baz.
We should figure out which notification types are affected and what we want their subject lines to be. Moreover, we may or may not want to consider rewording some subject lines to work around an RTL bug in GMail: T130578: Incorrect order of elements in email subject when username is RTL.