Welsh is entirely phoentic. With the exception of y, and vowels in diphthongs (2 vowels together), letters are always pronounced the same. And once you know the diphthongs, they are always pronounced consistently.
So is Czech. As are German, Polish, and Dutch, and probably most European languages with the exception of English and Russian. But the point is that the sounds assigned to each letter are broadly the same in each (OK, y and z are exceptions) and so pronounciation is clear and obvious. This isn't the case with Welsh, and the sounds of words seem seem very much at odds with their spelling and the letter combinations alien.
Had a bit of a dig and the Ll is discussed here and makes it pretty obvious why it's been Latinised to Ll, and L is used in other languages for it too; that the welsh seem to tell the English to use Sh and Scots to use Ch (the one from Nicht not Loch) is another matter, they're clearly just trying to give us a chance!
https://en.wikipedia.org/wiki/Voiceless_dental_and_alveolar_lateral_fricativesI don't see where the concept that the sounds don't match the spelling comes from though
https://en.wikipedia.org/wiki/Welsh_orthographyf is an english V
and ff is a english F
the only one that makes limited sense to an English speaker is dd being th (or Sh if you're from Fife but that could get confusing...)
more to that, you say that the "But the point is that the sounds assigned to each letter are broadly the same in each "
German V is close to English F
And German W is close to English V
Hence angry German bloke saying "Es ist Fow Vay nicht Vee DubleYoo"
Then you have G
In Dutch it's a velar fricative either voiced or unvoiced,
But in Swedish a G can be a /g/ or a /j/ depending on the letter that follows it. as in Göteborg
The madness of english has a fair bit to do with it having influences on top of the base Germanic of French, Norse, Norman French, Gaelic, Welsh, Hindi...
The 3 major language groups on Europe, German, Romance and Slavic are quite modern and importantly were not particularly insular as they met each other on the continent, the Celtic languages on the other hand were quiet insular until very recent times though you can see the impact of Norse and Latin on Gaelic quite clearly if you're looking for it, maybe the Welsh protected their language as part of their fierce protection of it against the wishes of English rulers?
https://en.wikipedia.org/wiki/Indo-European_languages
