NOTAMs are promulgated for the benefit of the flying community who should be able to interpret the content.
AAIB bulletins are published for the benefit of the flying community and also the general public, who cannot be expected to be able to interpret NOTAMs, hence the need for an interpretation of the contents of the relevant NOTAM(s).
The benefit of the current form of NOTAMs is that the original NOTAM is understandable to everyone irrespective of their native language. If NOTAMs were written in plain language how much room for mis-interpretation would exist for readers who did not speak/read/understand the language used.
How many English speakers would understand the following "In la région d'Information de vol Londres un obstacle a été érigé en affectant instrument et trafic visuel. Trafic Aereodrome et enroute est affecté. L'obstacle de la surface à 800 ft amsl et est placé dans un rayon de 1 mn 51 29 N 07 000 w. L'obstacle sera en place à partir de 1700 hrs sur 7 janvier 2013 à 2359 hrs sur 15 mars 2013. C'est une grue de potence de grande hauteur" if the original was published in French?