Parts of print/export dialog are not translatable

[eloquence]

Description

The initial headings in the print/export dialog are not translatable, even though translations exist in Weblate

Steps to Reproduce

1. Start the app with changes in feat: enable pt_PT and zh_Hans as supported languages #1531 using LANG=pt_PT securedrop-client or similar.
2. Initiate a print or export dialog

Expected Behavior

Entire dialog is translated

Actual Behavior

Only parts of dialog are translated:

Comments

This appears to be due to the common issue where string substitution is applied to a string prior to passing it to the translation function, which means that the string content is not constant, and translation lookup will fail.

securedrop-client/securedrop_client/gui/conversation/export/print_dialog.py

Lines 35 to 39 in 65d3541

	self.starting_header = _(
	"Preparing to print:"
	"<br />"
	'<span style="font-weight:normal">{}</span>'.format(self.file_name)
	)