Why do we say mother tongue and not mother language and why honeymoon and not honey month?
Honeymoon is a compound noun, meaning a holiday spent together by a couple immediately after their marriage.
We also have the expression honeymoon period, meaning the beginning of a period of time when everything is pleasant in a relationship and partners don't criticise each other:
They plan to go on honeymoon to Thailand for a month.
The honeymoon period for this new government is now over.
Honey month is an impossible combination and would not make any sense now, even though the word honeymoon was originally used to describe the first month of marriage. The reference to the moon (and therefore lunar month) is ironic: everybody knows that as soon as the moon is full, it starts to wane and dies.
Mother tongue - native language
Mother tongue is another fixed collocation. You are right, we do not say mother language. Instead, we would normally say native language, though native tongue is also possible, see below:
Her mother tongue was Russian, but you would never have guessed it from her perfect pronunciation of English.
You should acquire a perfect grasp of your native tongue before you start to learn a foreign language.
The greater part of learning a foreign language, is all about knowing which words naturally occur together. The examples given so far are relatively straightforward but it becomes more complicated when we look at the words which small and little naturally occur with