Lacking even least sign of language (5)
I believe the answer is:
latin
'lacking even least sign of language' is the definition.
'latin' can be an answer for 'language' (Latin is an example). I'm unsure of the 'lacking even least sign of' bit.
'even least sign of language' is the wordplay.
'even' means one should take alternating letters (letters in even-numbered positions are used).
'sign of language' becomes 'in' (I can't justify this - if you can you should believe this answer much more).
The alternate letters of 'least' are 'lat'.
'lat'+'in'='LATIN'
Can you help me to learn more?
(Other definitions for latin that I've seen before include "Cicero's tongue" , "Language of old Romans" , "A Romance language" , "Learning it 'an honour' (Churchill)" , "foreign language" .)