Does "athro cymraeg" mean a professor of the Welsh language or a professor who is Welsh?
Cymraeg means Welsh as regards to the language, so Athro Cymraeg is a professor of the Welsh language, while Cymreig mewns Welsh without regard to the language, so Athro Cymreig would be a professor who came from Wales.
That's what I thought but "professor of Welsh" was not accepted, so I reported it. Thanks!
Here it probably means 'teacher of Welsh', rather than 'Professor' in the British sense. Lewis was a lecturer in Welsh at The University College of Swansea in the 1920-30s.