This is a very broad question and you didn't provide much information about your game.
You can keep three copies of each key with language suffix, for example:
Or you can store all three values in one key:
Then you'll need a function that will parse this string and return the right value for each language.
As for the second question, it's better to use smaller sprites, tiled backgrounds and text objects for text. If you make each page as a big sprite, it will use a lot of memory!
See this blog post from Scirra:
https://www.construct.net/au/blogs/cons ... memory-796