“It is a great help for the person who is learning a foreign language to know some customs and manners for the people who speak it .”中文翻译,

对于学习外语的人来说，了解一些说这门外语的外国人的风俗和礼仪是有很大帮助的

it is great help for :for who ?
for the person who learing a foreing language !
then what is helpful ?
to know some customs and manners for the people who speak it !!!
nots: customs 风俗 manners: 礼仪
这样应该比较清楚了吧

想要更好的掌握你所学习的外语,去了解当地人所遵行的日常习俗和礼节
这个句子可看成It is a great help 4 the person后面的who is learning a foreign language修饰person,2 know some customs and manners 4 the people who speak it是实际主语,itis是形式主语,4 the people who speak it是修饰2 know some customs and manners .
整个句子可简写成2 know some customs and manners 4 is a great help.

了解一些原语种国的习俗和礼仪，对一个学习那个国家外语的人来说，是有很大帮助的