"Mae ffrog briodas Alys yn brydferth."
Translation:Alys's wedding dress is beautiful.
Note: Some of these forms may be hypothetical. Not everypossible mutated form of every word actually occurs.
both are acceptable: it's also common style to use apostrophe-s for words that already end in an S, especially with personal names such as Alys. though this can be wildly dependent on historical/local tastes (eg. it's St James's Park in London but St James' Park in Newcastle), and no-one recommends using it when you wouldn't pronouce the additional s ("Jeff Bridges' role in the Big Lebowski").
and of course it is never the case with plural nouns ending with s ("the girls' classroom")
Ligne. Thank you I see what you mean. Then how would a soldier be described who is wearing his ‘dress uniform’ ? If gwisg means uniform, wear, clothing, attire, etc then perhaps gwisg priodas wouldn’t be wrong. However, I completely understand that Ffrog priodas has now been accepted as the translation.