Relevant language — English? (7)
I believe the answer is:
germane
'relevant' is the definition.
(germane can mean relevant or pertinent)
'language english' is the wordplay.
'language' becomes 'german' (German is a language).
'english' becomes 'E' (abbreviation as in OED).
'german'+'e'='GERMANE'
(Other definitions for germane that I've seen before include "Related to the matter in hand, relevant" , "Relevant - appropriate" , "closely related" , "Relative or appropriate" , "Relevant (to subject)" .)