"I will go to New Zealand tomorrow."
Translation:A i i Seland Newydd yfory.
That makes sense. But it still strikes me as strange that a consonant has not been thrown in between the two "i's" to make them slurable, as there are so many other mutations in the language to enhance smoothness of pronunciation. Or maybe the mutations are not for ease of pronunciation but for some other reason...?
You're correct, mutations are there to make the language flow and it does make a difference. The two 'i', in the above sentence in the future tense and also in sentences in the past tense eg 'Es i i Gaerdydd' -' I went to Cardiff' and 'Es i i'r parti' - 'I went to the party' etc., are very easy to say quickly and meaningfully, especially with that short pause, or even a micro-pause, between them.
There are two aspects to mutations:
The majority are because the mutations make the words flow better together, or sometimes because they used to make them flow together better in earlier version of the language.
A minority are for grammatical reasons, again probably dating back to earlier versions of the language.
The mutation of adverbs such as doe -> ddoe (doe no longer seen in its unmutated form), ddydd Sadwrn (on Saturday), gwelais i fe *rywle. (I saw him *somewhere).
The objects of short-form verbs, such as in gwelais i gar (I saw a car), fe wnes i fynd (I did go -> I went).