JMSTranslationBundle not extracting 'placeholder' elements
I finally found out !
I used the latest stable version which is v1.1.0 as advised in the Installation docs
Please replace dev-master in the snippet above with the latest stable branch, for example 1.0.*.
so I added the version ~1.1 into composer. Unfortunately this version is already 10 months old and doesn't include the latest merges which addresses the placeholder issue (that was 9 months ago) !
So if you need to extract placeholder elements, use the dev-master version instead of the recommended "latest stable" ~1.1 !
I hope this post will save you the urge of ripping your own head off after more than 8 hours of search ;-)
Happy coding !