I've been always thinking that different individual has different opinions on this topic because everyone has different levels of abilities to master one language or many languages.

I'm learning Finnish, Mongolian, Manchu, Inupiaq, Vietnamese, Malaysian, Korean and Japanese. Each of them has different difficulties, and here's my opinions

I think grammatically speaking, Inupiaq is the most difficult in all of these languages I'm now attempting to learn and the second most difficult is Finnish; Finnish has 14 noun cases but fortunately most of these cases stand for direction not grammatical meaning; about Inupiaq, it's an "ergative-absolutive" language, I don't feel very familiar with this system so it's kind of hard to me, plus, the verb conjugation is weird because it consists of transitive conjugation and intransitive conjugation

Vietnamese, Korean and Japanese are "relatively" easier for me to learn because they were all affected by ancient Chinese language a lot, so I find it a piece of cake to master their vocabularies;

Mongolian and Malaysian are hard, too; they're both different from other languages I'm learning, so different language systems, different vocabularies.

I'd like to give lectures in front of a bunch of people, however, I have stage fright, hehe, sometimes when I say something in public, I usually forget words I need to deliver and I have weird pronunciation as well, so I really can't speak in public even though I do want to

I majored in English, so usually English graduates in China have jobs as teachers or export businessmen, I can't work as a teacher because of my shyness in front of people. O(∩_∩)O~

My mother language is not German, is southwestern Mandarin (a branch of Mandarin), and English is my second language

I'm learning Finnish, Korean, Japanese, German, English, Vietnamese; it's nice that I can find a place to practice

Turkishdictionary.net

If you just want to improve your colloquial Mandarin, then I guess it's totally ok to learn pinyin only since many people only learn Romanised arabic instead of Arabic inscript.

@Michelle

There are many Chinese languages used in mainland, it's impossible for every Han Chinese(汉族) to speak Standard Mandarin/Putonghua "correctly", because putonghua is nobody's mother language, everyone has different local mother dialect/language, just like I'm now in Shenzhen, Guangdong, I hear different tongues everyday, Sichuan dialects(四川话), Hakka(客家话), Teochew(潮汕话), Cantonese(粤语/白话), Gan language(赣语), Lei Min language(雷州话) and some from my hometown as well;

even in northern China where it's thought to be the origin of Putonghua, people still speak Putonghua with local accents, beijingers use "-er“ a lot and they also have dialectal words and phrases;

I don't really think anyone in China uses Putonghua as mother tongue untill now our "dear" government encourages every child to use this tongue, which has caused many children to speak Putonghua with dialectal accents because their parents or teachers don't really know how to speak this common tongue properly.