I know that this issue has been dealt with several times in this forum
I have installed the Italian language pack on my Magento 2.1.9 project and most strings are correctly translated.
I added a new translation dictionary in my theme.
Here I've modified some translations and added the new strings of my theme to translate. For strings included in phtml files everything works fine.
I added a some custom strings in knokout templates of checkout. For example in
<span data-bind="i18n: 'Do you already have an account?'"></span>
Then in my theme it_IT.csv:
"Do you already have an account?","Hai già un account?"
Of course I emptied cache, pub/static/, var/view_preprocessed/, var/generation, etc.. and I runned all the commands.
The strings are still in English.
I really have to create a new language package to translate a few strings in the checkout? What's the reason?
EDIT Online translations work for all strings. It can be a way to fix it temporarily but not the definitive solution.