"But ministers said it was not unprecedented for similar legislative orders to be approved shortly before elections and the same had happened in the run-up to the 2010 general election." Doesn't mean it's right or sensible though! We had last minute legislation in 2004 as well - in that case significant guidance (if not actual regulations) we still being written after the publication of the notice of election.
There has been a persistent pattern in recent years of changing/finally agreeing details of election rules at a very late date (ie after the election period has actually started). There aren't vast numbers of Welsh only readers but there are some - and there are undoubtely more who read Welsh much better than English. So generally speaking government publicity material in Wales is produced in a bi-lingual format. I can't t see why this hasn't been dealt with much much earlier.
Often it's better to do what you think is right and seek forgiveness afterwards. How bad / costly would it have been to just print them in both languages anyway? Are English-only pamphlets really of no use at all? Are there many Welsh-only speakers? I do think this is ridiculous all round. £350,000 would have bought a lot of school books and equipment.