Mandarin is a very hard language to learn. Party because it is tonal. One word can be said four different ways. To learn to hear that tone and be able to make the tones is a great thing. Many adults can't do it myself inc. Another reason is the sentence structure is very different. Ie we would ask "what do you want to eat?" in Mandarin you ask "you want eat what?" so the brain switches around a lot. Once you get round the grammar and can hear the tones it gets easier. My kids don't have much difficulty switching between languages but the 1st year the start school (school is in Mandarin) they have a few mistakes.
Mandarin is in the top 3 most spoken languages.
European languages are much easier to learn as an adult and perhaps easier for you to help with.