"Wales is in Britain."

Translation: Tha a' Chuimrigh ann am Breatainn.

[KigeliaAfricana]

shouldn't this be: anns a' Bhreatann (if Breatann is feminine with a definite article)

[silmeth]

If it were with a definite article – yes, regardless whether it’d be feminine or masculine (the definite article always lenites in dative singular, see eg. anns a’ bhàta ‘in the boat’ or ris an t-sagart ‘with the priest’, but yes, Breatainn is feminine).

But Breatainn Britain is used without the definite article, hence just ann am Breatainn, there’s no definite article here, just like in English in Britain.

What I’m not sure is whether it should be Breatann or Breatainn though – as dictionaries (Am Faclair Beag, Colin Mark’s) give it with broad consonant for nom./dat. (Breatann) with slender in genitive (Breatainn for Britain’s, of Britain), but this course and Wikipedia seem to use the slender ending (Breatainn) for nom./dat.