这是您可以做的，

按语言搜索，然后按标题&过滤作者

For search by language and then filter by title & author

public void search(String languangeSearchString, String title, String author){  
        FirebaseDatabase.getInstance().getReference().child("Posts").equalTo("language").startAt(languangeSearchString).addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                 String id = dataSnapshot.getKey();
                if(dataSnapshot.child("title").exists() && dataSnapshot.child("author").exists()){
                    if(dataSnapshot.child("title").getValue(String.class).equals(title) && dataSnapshot.child("author").getValue(String.class).equals(author)){
                        //Here are the results
                    }
                }           
            }

            @Override
            public void onChildChanged(DataSnapshot dataSnapshot, String s) {

            }

            @Override
            public void onChildRemoved(DataSnapshot dataSnapshot) {

            }

            @Override
            public void onChildMoved(DataSnapshot dataSnapshot, String s) {

            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });

}