Background:
I\'ve written a backup tool for the Mercurial/Git hoster Bitbucket, which asks for a username, gets a list of the user\'s repositories via the Bitbucket API,
I suspect the answer is No, there's no standard. Here are some options:
Bugzilla can import/export both CSV and XML:
Here's this search: https://bugzilla.mozilla.org/buglist.cgi?quicksearch=css3+text exported to CSV and XML and here's this bug: https://bugzilla.mozilla.org/show_bug.cgi?id=156881 exported to XML.
JIRA can import from Pivotal Tracker, Bugzilla, FogBugz, Mantis, Trac and more, apparently, and can export as XML and CSV.
Here's this bug: https://jira.atlassian.com/browse/WBS-49 exported as XML and this search: https://jira.atlassian.com/browse/GHS-5597?jql=text~%22greenhopper%20api%20plugin%22 exported as XML - you can also export these as .xls files.
The roundup-admin CLI has a sub command export which can be used to create a bunch of .csv files together with sub directories containing the messages and file attachments. See http://roundup.sourceforge.net/docs/admin_guide.html#migrating-backends for how this can be used to migrate the issue data to another database.
Trac can export to CSV & TSV. Here's this search: http://trac.edgewall.org/report/2 exported to CSV and this bug: http://trac.edgewall.org/ticket/886 exported to CSV and TSV.