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Some details available in JSP 752 Chapter 9 Section 13 'Language Awards' available online
here through the RAF Community Support
website.
09.1301. Aim. The aim of Language Awards is to offer a financial reward to encourage Service personnel to obtain language qualifications that are directly related to the needs of the Service. The detailed rules and procedures, sponsored by SP Pol Manning, regarding the eligibility and entitlement to Language Awards are laid down in the following single Service regulations:
a. RN - BR 1797 Chapter 16.
b. Army - AGAIs, Volume 1, Chapter 21.
c. RAF - AP 3379, Volume 2, Leaflet 1919.
09.1310. Eligible Languages. The operationally important languages covered by the Scheme are Arabic, Farsi/Dari and Pushtu. Awards will be paid to Service individuals who apply to join the Scheme and are accepted by passing the MOD Language Examination Board (MODLEB) examination at the required standard in any of these languages.
There's more at the link.