I’ve added some data to Kynulliad3 (it’s not in the current download, but it will be in the next one) so that the “language of first delivery” can be calculated. In the Assembly, Members can speak in either language, which is placed on the left-hand side of the Record. The translation, into English or Welsh as appropriate, then goes on the right-hand side of the Record. If we separate out those sentences which were first delivered in one language from those which have been translated from the other language, we get the following word totals:
Welsh: 879,964 / 8,865,452 (9.9%)
English: 7,916,148 / 8,775,382 (90.2%)
So about 10% of the word total in the Third Assembly used Welsh as the language of first delivery. This is a bit lower than the proportion of Welsh-speakers in Wales (19% according to the 2011 Census), but I don’t know how it relates to the proportion of Welsh-speakers in the Third Assembly (or at least the proportion who felt confident enough to use Welsh in this setting). It’s a pretty sizeable percentage, though.