"Plaid Cymru is the youngest."

Translation: Plaid Cymru yw'r ifanca.

[SeanDamienF]

Whatbs wrong with "Mae Plaid Cymru yn ifanca"?

[ibisc]

See the notes on the section 'Comparing3' earlier in the course.

When expressing the superlative '-est' forms in Welsh we use an emphatic construction, so the thing that is -est' comes first in the Welsh sentence.