Hi iger,
What do you mean by "the default language"?
Are you speaking of the text that is shown when you open a template containing text boxes, like this sample from the Informal Newsletter template?

That's "placeholder text", and it's there to show what the layout will look like when it's filled with text. It's in Latin mostly to allow you to concentrate on what the text looks like, rather than what it says.
To replace it with your own text (in whatever language you choose, Click in the box:

Note that no matter where you click, all of the placeholder text is highlighted, and with your first keystroke (or with a paste of text copied from somewhere else), the palceholder is replaced by what you type (or paste), in whatever language you type it:

Regards,
Barry