Possibly related to Bug 552158.

In the help/LL/ subdirs a .xml2po.mo file is produced with each invocation of xml2po with -p flag. During the same invocation translations from that file get merged into the target xml.

Now running make -j5 a race condition exists where one make job can overwrite .xml2po.mo in the lang directory if more than one source file exists (multiple mallard pages or multiple docbook_includes / entities).

This error is reproducible with random results. All depends on whether make managed or not overwrite .xml2po.mo or not and at what stage of xml2po run.

Possible solutions:

1) Create LL/.xml2po.LL.mo and reuse it in all subsequent xml2po invocation with -t flag instead of -p flag (more efficient as long as all translations are aggregated from all entities/pages)

2) Use mktemp for aneming .xml2po.mo (then the bug is in xml2po)

I'm not sure which one is better. I'm not an expert. And sorry I can't provide a patch make is very scary language. I use waf ;-) and discovered same issue cause I was mimicking make behaiviour except that waf runs in parallel by default hence I discovered this issue.

Sorry I'm behind low bandwidth and can't clone git repo to test using head.
Example errors using gnome-doc-utils 0.18 of two different projects:

With docbook:

if ! test -d fr/; then mkdir fr/; fi
if [ -f "C/onlinehelp.xml" ]; then d="../"; else d="/home/dmitrij/src/upstream/xiphos/trunk/help/"; fi; \
	po="fr/fr.po"; \
	if [ -f "${po}" ]; then po="../${po}"; else po="/home/dmitrij/src/upstream/xiphos/trunk/help/${po}"; fi; \
	(cd fr/ && \
	  `which xml2po` -m docbook -e -p "${po}" \
	    "${d}C/onlinehelp.xml" > onlinehelp.xml.tmp && \
	    cp onlinehelp.xml.tmp onlinehelp.xml && rm -f onlinehelp.xml.tmp)
Traceback (most recent call last):

+ Trace 219945

Or with mallard:

if ! test -d el/; then mkdir el/; fi
if [ -f "C/geolocation-supported.page" ]; then d="../"; else d="/home/dmitrij/src/empathy/help/"; fi; \
	po="el/el.po"; \
	if [ -f "${po}" ]; then po="../${po}"; else po="/home/dmitrij/src/empathy/help/${po}"; fi; \
	(cd el/ && \
	  `which xml2po` -m mallard -e -p "${po}" \
	    "${d}C/geolocation-supported.page" > geolocation-supported.page.tmp && \
	    cp geolocation-supported.page.tmp geolocation-supported.page && rm -f geolocation-supported.page.tmp)
Traceback (most recent call last):
  File "/usr/bin/xml2po", line 185, in <module>
    main(sys.argv[1:])
  File "/usr/bin/xml2po", line 171, in main
    xml2po_main.merge(mofile, filenames[0])
  File "/usr/lib/pymodules/python2.6/xml2po/__init__.py", line 601, in merge
    self.gt = gettext.GNUTranslations(mfile)
  File "/usr/lib/python2.6/gettext.py", line 180, in __init__
    self._parse(fp)
  File "/usr/lib/python2.6/gettext.py", line 273, in _parse
    magic = unpack('<I', buf[:4])[0]
struct.error: unpack requires a string argument of length 4
make: *** [el/audio-call.page] Error 1
make: *** Waiting for unfinished jobs....