Move localized prompts into a separate file

This pull request moves localized prompts into a `localization.js` file. This simplifies the addition of more languages in the future and cleans up the code for HTTP routes. Moving localization data into its own file will also prevent future oversights or mistakes such as the one seen here: 267ca8159372800aabd37239505c23098145b6ed (This pull request conflicts with the currently open pull requests #33 and #32.)