My prefix weighs little, but never has mass;
My suffix might, for a whole language, pass;
But to me append,
One more suffix at end;
And scramble my innards so I'm longer at last
What am I?
The answer is:
LIGHTEN
My prefix weighs little, but never has mass;
Something which is LIGHT 'weighs little'. Also, light (the noun) has no mass.
My suffix might, for a whole language, pass;
EN is an abbreviation for 'English', often in language settings on computer programs and websites.
But to me append,
One more suffix at end;
And scramble my innards so I'm longer at last
Add the suffix ER to get LIGHTENER then anagram (scramble) the inner letters to get LENGTHIER, meaning 'longer'.
As for the title:
'Lighten up' and 'cheer up' can be synonyms, as in how a drop of sunlight can 'lighten up' a drab room - or as in 'Lighten up, dude!'