"let in" is correct and usable in written English. It is usually used to indicate either allowing someone to enter or to express one's openness and willingness to accept something. Example: We will let in anyone who arrives before 8pm.
I let in a lot".
Too many people let in, it seems.
Only newly-promoted Bournemouth have let in more.
Seal gaps that let in cold air.
Let in.
I wasn't let in.
And darkrooms let in light.
"We wish we could let in more".
Greece should never have been let in.
I let in all their runs".
So I was not let in.
Being a terminologist, I care about word choice. Ludwig simply helps me pick the best words for any translation. Five stars!
Maria Pia Montoro
Terminologist and Q/A Analyst @ Translation Centre for the Bodies of the European Union