According to a friend of mine who speaks Welsh, it's actually very simple.

Because the Latin alphabet was used to transcribe the language phonetically, there is
one way to write down spoken language and one way to pronounce the written form.

The only problem for English speakers is that to cover all the phonetic components of
Welsh, letter combinations are used, not just single letters and they mostly represent
*different* sounds than in English. Hence, the pronounciation is not what an English
speaker expects.