Hi learner,
since I have the same problem I have asked the Austrian CAA about the recognition the endorsement "English proficient" within an FAA licence. I was told that this kind of endorsement would not corresspond with the wording rcommended by ICAO, i.e. it does not clearly show the validity of language level (see also
ICAO | FLS | FAQs ). Moreover, they told me that "English proficient" was not equal to Level 6 and I was made aware of FAAs Info 08012 (see
http://www.faa.gov/other_visit/aviat.../info08012.pdf ). On page 2 (first paragraph) it says "The FAA’s issuance of an “English Proficient” endorsement on a U.S. airman certificate attests that the airman meets the Level 4 operational standards found in ICAO Annex 1...". Therefore, they were not able to accept the endorsement...I think with the Dutch authority it should be the same. Allegedly, other ICAO member state the validity in there licence...Sorry, that this does not help you, but at least it gives an explanation.
Take care,
aileron