Translation:I teach Welsh in a school. I'm a female teacher.
‘I’m teaching Welsh’ is fine too. It denotes something temporary but it’s a legitimate sentence.
Welsh doesn't differentiate between between "I something" "I am something-ing" as far as I know. So this is true of most sentences. e.g. Dw i'n mynd = "I go" or "I am going".
How come I got counted wrong for "I learn Welsh in school" as opposed to "... in A school"? I know it's supposed to be "teach" here, not "learn," but the only thing underlined as incorrect was "a." Is it sort of a British/American distinction? Like how the British "in hospital" sounds weird to us Americans, since we always say "in A hospital"...