Yvonne Evans, Welsh second-language teacher at Llangatwg comprehensive, in Neath, said: "It is complete immersion, and I could see a difference in my skills after just one week. We are also learning teaching skills."
TES Cymru reported on plans to launch the pilot sabbaticals last May.
Around 160 places were promised in the first 18 months of the pound;2 million-a-year scheme. Courses are run at Cardiff and Bangor universities.
Speaking at the official launch of the Welsh sabbaticals this week, education and lifelong learning minister Jane Davidson said: "I would encourage teachers to sign up now and help us on the road to a fully bilingual Wales."
Applications close February 17.
* Only four out of 10 secondary schools reflect the language and culture of Wales in lessons, according to Estyn. Its annual report says primary schools are getting better at teaching Y Cwricwlwm Cymreig, but knowledge and understanding of Welsh culture and history is not so consistent in secondary schools.