The most common question among people with a passing interest in language learning is probably “What is the easiest language to learn?” It`s a good question, but the answer is not the simple one most people are hoping for. That`s because it depends on you and what your native language is, and on whatever other languages you speak.
Similarity to Your Native Language
Basically, the more similar a language is to your native language, the easier it is for you to learn. Assuming no previous exposure to any foreign language, the easiest language for a native Korean speaker to learn is most certainly Japanese, because their grammatical structure is extremely similar. The easiest language for a native Malay speaker to learn is absolutely Indonesian, because they are grammatically identical and 80-90% similar in terms of vocabulary (they probably don`t even need to study it, they just need to get used to the vocabulary differences and accent). What language would be easiest for a native Hindi speaker? I suppose Urdu, or Punjabi if you consider Hindi and Urdu to be the same language. So there is no single language that is easier to learn for all people around the world.
Similarity to Other Languages You Have Learned
A language may also be easiest for you if it is closely related to another foreign language you have already learned. If you are not a native French speaker but you have learned French to an advanced level, then the easiest foreign language to learn is probably Italian, because of its largely cognate vocabulary and closely similar grammar. I think there are languages that are more similar to French than Italian is, but they are minority languages in France and there is likely little motivation for non-native speakers to learn them. Motivation is a factor in how easily you can learn a language.
When you learn a foreign language, you are learning new linguistic concepts that don`t exist in your native language. It is these differences that take up the bulk of your learning time and effort. So if you learn a second foreign language that is similar to the first one you studied, then there will be a lot of concepts that are familiar to you. Learning those concepts take a lot of time and effort, but you already know them! So you can breeze through them, making the language much easier to learn.
Simplicity of the Language
The simplicity of a language is another factor in how easily you can learn it. Simplest doesn`t necessarily mean easiest, because the language may be completely unrelated to your native language. A complex language that`s closely related to your native language might be easier to learn than a simple one that is completely unrelated, because many concepts and vocabulary might be familiar. But simplicity is a factor.
So far for me personally, I have found French to be the easiest language to learn to communicate in, because of its huge shared vocabulary with English, but French has a complex verbal system that slowed me down. The easiest language to grasp in terms of grammar and sentence structure has been Indonesian because of its simplicity, particularly its simple verbal system which requires no conjugation for tense. It didn`t take long for me to be able to confidently make grammatically correct sentences in Indonesian.
The biggest factor in what language is easiest for you to learn is its similarity to your native language. But its similarity to other languages you know, and its simplicity are also factors.
See also: What is the Hardest Language to Learn?