1) For non-native users, they spend comparatively more time in acquiring English and thus comparatively less time for their native language so the proficiency in the native language weakens because of the learning of English.
2) Some people use a mixture of English and their native language when they speak. For example Spanglish (Spanish + English). This is actually due to the inflexibility of the brain to separate the 2 languages so it thinks semi-English and semi-native way. This hinders higher order mastery of English