I am a speech pathologist and I thought I should reply to give some suggestions.
The general benchmark for a child to acquire his/her first word is around 12-24 months. From my experience, by 16 months the child should have his first word. It should not be something that the child imitates from what you are saying, but something that the child says independently and knows exactly what he is referring to. So in short mama/dada doesnt count, ball can could if the child says it independently to request the ball without you modelling the word ball.
Someone suggested to use PECs but again, the child should be reviewed by a speech language pathologist to deem the feasibility of this. I am not a fan of PECs as it is rote learning and highly dependent on rewards. the child will be 'programmed' to request because he knows that if he point to the right picture and give it to the adult, he will get what he wants but he don't know why he is doing this. In other words, communication isn't meaningful to him, we want the child to want to communicate because the child is interested in you or the thing. There are other free things out there that can be used to build a child's language. Something like an aided language stimulation board/core language board will be useful. For children who are still learning how to talk, meaning they are only at one word level, using these boards might be useful as you introduce new words to the child and model how to use this words. In time, the child will know oh mummy is pointing to eat apple, this means that she wants me to eat the apple, I will eat the apple now. With frequent modelling, the child will then independently use it on his own. There are consistent research out there that shows that using a language board for children who are still developing talking skills
WILL NOT hinder speech and language development. In fact, there are research that shows that it actually helps to expedite the process.
There are some children that are late talkers, meaning children who do not say their first words until they are 3 years old and suddenly have an explosion of language skills. Boys are more likely to be late talkers but do take note that the cohort of children that are not talking, only around 20% are considered late talkers while the remaining 80% are language delayed/disordered. Hence early intervention is the key and I would definitely suggest not to wait it out as the child could miss critical milestones that are essential for communication development. A good app that I highly recommend to parents to track the child's developmental milestone is developed by the Centres for Disease and Control and Prevention's Learn the Signs and act early program. It is free and allows you to track your child's milestone and if it is not age appropriate will let you know to seek services.
https://www.cdc.gov/ncbddd/actearly/milestones/index.html
There are various language modelling techniques that are useful for children who are language delayed/disordered or even for typically developing children that you want to boost their language skills. Since speech therapy may be costly or has a long waiting list, I would suggest you all to use these if you are concerned while waiting for these services. Again, I don't know any of your children and so these are suggestions and not absolute directions. Every child is different and their language learning progression is different.
1) Parallel and Self Talk
These are by far my favourite and I always recommend to my parents. Basically Parallel talk meaning that you are narrative what your child is doing while self-talk you are narrating what you are doing.
Parallel Talk eg (your child is playing with a ball)
Child: (actions bouncing the ball)
Mum: Oh Daniel, look at you, you are bouncing the ball. Bounce bounce bounce!
Self Talk (in a play activity with bubbles)
Mum: Mummy is going to blow the bubbles. I am going to blow a big one. Ready set....go!
2) Commenting rather than questioning
Before a child can have the ability to answer the questions, he first has to be in a language rich environment where the people around him are using self talk, parallel talk, descriptions and comments so he can learn the vocabulary and language first. Though it is definitely OK to ask your child questions sometimes that they cannot answer- especially of infants and toddlers (while then providing them the answer right after) you want to balance this with questions that he/she can answer without help while continuing to provide your child with good language models through the use of the strategies already mentioned.
Sometimes, we all fall into the trap where start questioning the child. Questioning is especially difficult for a child who has difficulties talking and it actually makes him stress and might prevent him to communicate further because he knows that he needs to reply but he cant reply. e.g. "Daniel what colour is this? "Daniel what do you want to eat etc."
Commenting is much safer because you are just making an opinion on something and it is not required of the child to reply. Therefore the key is to use think alouds like ohhhh I wonder.... going back to the colour eg, I wonder what colour is it? and you can follow up with a forced choice is it blue or is it red? With these options, the child is likely to reply and engage with you!
Feel free to ask any questions, happy to answer!