1. https://effectivelanguagelearning.com/language-guide/language-difficulty/
2. https://taleninstituut.nl/the-hardest-languages-in-the-world-to-learn/
3. https://list25.com/25-of-the-most-difficult-languages-to-learn-in-the-world/
4. https://www.ranker.com/list/hardest-language-to-learn/jennifer-lee
5. https://www.getthatright.com/difficult-languages-to-learn/
6.https://resources.unbabel.com/blog/japanese-finnish-or-chinese-the-10-hardest-languages-for-english-speakers-to-learn
7. https://www.thetoptens.com/hardest-languages-learn/
8.https://resources.unbabel.com/blog/japanese-finnish-or-chinese-the-10-hardest-languages-for-english-speakers-to-learn
9. https://inspirationfeed.com/hardest-languages-to-learn/
10.https://www.scientificamerican.com/article/at-what-age-does-our-ability-to-learn-a-new-language-like-a-native-speaker-disappear/