Besides the mother tongue, the easiest languages to learn are those that most closely represent your mother tongue.
I live in Japan and the sentence structure is so dissimilar and so I find it difficult. Also they have three writing systems Katakana for foreign words, Hiragana for Japanese words and Kanji which are the symbols they used. The first two are relatively easy and it took me two months to master. However, when I asked my native Japanese wife how long it will take me to learn Kanji studying an optimistically three times a week she told me ten years.
I have also studied Italian and I can recall more Italian than I can Japanese even though I have been here longer. The last time I studied Italian was about 12 years ago. I am British we use the same alphabet system. Other than the adjective following the noun the sentence structure is similar to English and as the English Language is rich in Latin words it also makes it easier.
So basically it boils down to what do you know and it is surprising that you may already have a good knowledge of a language before you start to learn it due to the similarities I have noted.