The WPML support said today:
ACF is not currently compatible.
And you should “hack” the database to fix it.
http://wpml.org/forums/topic/wpml-3-1-8-1-field-groups-not-translatable-acf-now-working/#post-498588
Draw your own conclusions…
Edit: The technical background is explained here:
http://wpml.org/forums/topic/broken-acf-languages-after-activate-plugin/page/3/#post-493055
Summary: WPML 3.1.7+ downloads a remote config from WPML server which currently tells WPML that ACF can not be translated. The update of this remote config is triggered at every after_switch_theme and activated_plugin wordpress action and at custom wpml-actions.
