This morning we had Hindustani exponent Sandhya Kathavate speaking on the above subject. As always I was late by ten minutes. So when I went in she was speaking on how thumris are invariably in braj bhASha. This she said is a very musical language as it has most of its words ending with vowels (like Telugu). There are other languages also in use such as khaDi bOli, bundEli, mAlwi etc. In the Laknavi variety (from Lucknow), we have a mix of Hindi and Urdu. Sometimes the asthAyi will be in braj and the antara in Urdu set like shairi.
Ragas used in thumris are lighter such as khamAj, tilang, pahADi, bhairvi, shivranjani and pIlu. Some allowance is also given to bringing in shades of other ragas to properly express an emotion. Thus you have instances of ragas such as mishra khamAj.
Some of the talas that are used include tIn tAl, dIpcandi, and something called addha if I am not mistaken.
She sang pieces right through, playing the tabla herself. Now I can well believe that Narayanaswami Appa accompanied himself on the mridangam while performing bhajans. She sang beautifully.