I guess it depends. If they have been developing mobile apps, and now want to develop a web app, then they definitely need to learn something like PHP, or Go or Python kr Java. On the other hand if they have been doing web development and now want to develop a native app, they must learn Java/Kotlin/Swift. Same for databases (perhaps you never worked with one, then you must learn sql). Even html+css must be learned if you never used them before.