Switch to translation files for internationalization

[tanius]

Currently, one src/assets/data/*.json file is used to provide the complete form for one language, so the form structure is redundantly contained in the .json files. Due to this, the danger of modifying one .json file and getting the settings / structure of the form out of sync with the other translations is somewhat high.

If this software starts seeing heavier use, it would be better to split the structure of a form (what fields it has, how it behaves, what fields are required etc) from the content (what the titles / buttons / fields say in whatever language is selected). This will allow us to throw a translation file up on something like Transifex and allow the community to update translations without needing any technical ability.

To minimize the development effort, this should only be implemented if and when switching to QML as the language to define the user interface.