This is a really interesting entry in the English Central blog by DDeubel.
This is what he says about the most important factors that influence learners when studying a second language:
"The biggest factor contributing to a language learner’s success at learning a second language is motivation.
Second is language distance – the difference from the target language and the amount of interference or transfer that can occur.
Learning English can be harder or more difficult depending on your mother tongue. Here are some infographics that detail the languages that have the least language distance (and thus, make it easier to learn English) and those with the most language distance. The photos list how difficult it is for a native English speaker but the case is the same in reverse – for a non-native speaker wanting to learn English!"