There is no simple way to do that.
The language Filemaker is working in depends on your system language. In OS X it'e enough to change the system language to have FileMaker using ContaRicorrenze instead of PatternCount, but that's not your problem.
When the interface has been done using static text, concrete labels written on layouts, then you will have to edit each single element and eventualy adapt the label's length, as "Data di nascita" is longer than "Birth date".
You can have Merge fields based upon a language table, the shortest form being <<|::|>>, in order to maintain both the english and the italian. But as philmodjunk says, you wiil have to act upon every single label, dialog box text, etc.
When in layout mode, you can find / replace a text with another, but it's a minimal help.
You might also use API calls to a site such as Google Translate to produce translated versions of the data in your fields, but this isn't "simple" and such translation software is far from perfect.