Language Pack for Thunderbird Lightning

manifest.json

{
  "languages": {
    "LANG": {
      "chrome_resources": {
        "calendar": "chrome/LANG/locale/LANG/calendar/", 
        "lightning": "chrome/LANG/locale/LANG/lightning/", 
        "gdata-provider": "chrome/LANG/locale/LANG/gdata/"
      }, 
      "version": "VERSION"
    }
  }, 
  "applications": {
    "gecko": {
      "strict_min_version": "MINVER", 
      "id": "langpack-cal-LANG@archlinux.org.community.xpi", 
      "strict_max_version": "MAXVER"
    }
  }, 
  "langpack_id": "cal-LANG", 
  "version": "VERSION", 
  "name": "LANGUAGE (LANG) Language Pack for Lightning", 
  "manifest_version": 2, 
  "sources": {
    "browser": {
      "base_path": "browser/"
    }
  }, 
  "author": "PACKAGER", 
  "description": "Language pack for Thunderbird Lightning for LANG"
}

PKGBUILD

# modify for your language as available at https://hg.mozilla.org/l10n-central/
_lang=de
_langname=German
_tbmin=60.0
_tbmax=70.0

pkgname="thunderbird-cal-i18n-$_lang"
pkgdesc="$_langname language files for Thunderbird Calendar"
pkgver=20180805092655+60.0+70.0
pkgrel=1
source=("manifest.json" "thunderbird-cal-$_lang::hg+https://hg.mozilla.org/l10n-central/$_lang")
sha256sums=('SKIP'
            'SKIP')
arch=(x86_64)

pkgver() {
  cd "thunderbird-cal-$_lang"
	printf "%s+%s+%s" $(date -u -d "$(hg log -l1 -T "{date|isodatesec}")" +"%Y%m%d%H%M%S") $_tbmin $_tbmax
}

build() {
	_src="thunderbird-cal-$_lang"
	langpath="chrome/$_lang/locale/$_lang"
	rm -r build
	mkdir -p build/"$langpath"
	sed -e "s/VERSION/${pkgver}/g" \
  	-e "s/MINVER/$_tbmin/g" \
  	-e "s/MAXVER/$_tbmax/g"  \
  	-e "s/LANGUAGE/$_langname/g" \
  	-e "s/LANG/$_lang/g" \
  	-e "s\$PACKAGER\$$PACKAGER\$g" \
	  manifest.json > build/manifest.json
	mkdir -p build/defaults/preferences
	sed -n "/@AB_CD@/d;/^pref/p" $_src/calendar/lightning-l10n.js >build/defaults/preferences/lightning-l10n.js
	cp -a $_src/calendar/chrome/* build/"$langpath"
	cd build
	mv "$langpath"/calendar/providers/gdata "$langpath"/gdata
	mv "$langpath"/calendar/providers/wcap/* "$langpath"/calendar/
	rm -r "$langpath"/calendar/providers
	bsdtar --format zip -c -f ../langpack-cal-$_lang\@archlinux.org.community.xpi.xpi *

}

package() {
	install -Dm644 -t $pkgdir/usr/lib/thunderbird/extensions/ langpack-cal-$_lang\@archlinux.org.community.xpi.xpi 
}

How to make a package or extension from it? I copied both files to a folder and run makepkg but got an error in build and since this should be .xpi file it should be done differently. How?