I dont understand why "Tapadh leibh Isabel agus Elizabeth.", is incorrect. A person's name is a proper noun so should be acceptable. I understand it's teaching what the equivalent name would be, but when you're saying thank you to a specific person you wouldn't just change their name to suit yourself.
As it’s explained in the tips and notes to the Names skill, in Scotland it’s actually pretty common to use the English variant of the name when speaking English or Scots and its Gaelic equivalent when speaking in Gaelic (esp. if the person has the English name as the officially recorded one but uses the Gaelic one everyday). So when speaking to a specific person you actually would ‘just change their name’ but not ‘to suit yourself’ but because that’s the common practice in this region and that’s what the person you speak to expects you to do.
Of course you probably wouldn’t do that when speaking to somebody from outside of Scotland – then you’d probably just use the form of the name they use. But the course shows you the typical Scottish Gaelic practice among native speakers and expects you to follow it.
See the notes:
It would generally be considered rude to translate a French name such as Pierre into Peter in English. The same is not true for Gaelic. Most native Gaelic speakers would be known by their Gaelic name in Gaelic, and its 'translation' in English. Someone known as 'Oighrig' in Gaelic would almost certainly known by its translation 'Effie' in English . We want to show learners what actually happens in Gaelic communities and so we have followed this convention.
Some Gaelic names such as Iain and Mòrag are so common in Scottish English that they are not translated in the course.
It is becoming increasingly common for parents to give children a Gaelic name as their given / recorded name, which is lovely.
You can read the notes on the web browser version of Duolingo at https://duolingo.com and also on the https://duome.eu/tips/en/gd website. Unfortunately they might not be available on the Duolingo mobile app, so if you’re on mobile you might want to open your web browser for reading.