In Welsh, the compass directions are treated as nouns, not adjectives. Putting two nouns together shows a possessive relationship between them, with the thing possessed coming first in the pair:

• car Dewi - Dewi's car (literally, in awkward English, 'the car of Dewi')
• tîm Cymru - the Welsh team (lit - 'the team of Wales')
• de Cymru - south Wales (lit - 'the south of Wales')

Note that Welsh does not need the equivalent if 'the' and 'of' in the English patterns.

So, and remembering that yn used as a preposition meaning 'in' causes a nasal mutation:

• yn ne Cymru - in south Wales