In order to enable Launchpad Translation support for your xpi based application package, your package has to produce a template during build. The template is a valid en-US.xpi file at best produced after build from the applications chrome files.
If you don't want to use mozilla-devscripts you need to produce a valid en-US.xpi (template) file with all the locale specific files from your install tree remember that you need a install.rdf to the produced en-US.xpi; in order to make launchpad automatically import your template, ensure that its placed in the debian/lp-export-xpis/ directory after build.
If you use mozilla-devscripts, there is some magic for you that filters out a valid en-US.xpi file from almost any package install tree. To use that you need to do a few things (see firefox-3.0 package for an example):
include /usr/share/mozilla-devscripts/lp-locale-export.mk in your cdbs-style like debian/rules
define at least the following variables:
MOZ_TRANSLATION_AUTO_XPI_OUTDIR := debian/lp-export-xpis/ MOZ_TRANSLATION_AUTO_XPI_SEARCHDIRS := debian/tmp MOZ_TRANSLATION_AUTO_XPI_LOCALE := en-US
ensure that you hook the auto-locale-xpi.mk/translate make rule in a post-install debian/rules target and remember to add a valid install.rdf to the produced en-US.xpi (see the firefox-3.5 package for an example), like:
cd debian && zip lp-export-xpis/en-US.xpi install.rdf