Challenges in learning a foreign language

What’s the biggest challenge that comes to the minds of students when they learn a foreign language? Grammar? Not enough vocabulary? Unable to understand native speakers who speak really fast or people with an accent? Lack of confidence in expressing themselves in a foreign tongue? It could be all of the above if we are talking about a new learner or a learner who needs to work a lot on their foundation. 

With the help of modern day tools such as mobile apps and exercises along with explanations on the internet that cover mostly grammar and vocabulary, learners only need to have the opportunity to practice with other learners or native speakers so that they can get to practice the language they are learning with people from different mastery levels of the language speaking in different accents.