You may use $nin Comparison Query Operator for this

Example:

Users
  .find({ user_enabled: true })
  .populate({
    path: 'books',
    match: { 'language': 'en' }, 
    match: { "language": { "$nin": ['xx','yy'] } }  // <= See here
  })
  .exec()

Users
  .find({ user_enabled: true })
  .populate({
    path: 'books',
    match: { 'language': {$not: 'en'} }, })
  .exec()

Have you tried $ne operator?

So in your case:

Users
  .find({ user_enabled: true })
  .populate({
    path: 'books',
    match: { language: { $ne: "en" } },
})
.exec()