Adding Dutch language to the app

[pavel-ilin]

fixes #595

Please provide enough information so that others can review your pull request:

• Create directory for Dutch language client/public/locales/nl with translation.json in it with Dutch language content.
• Add Dutch language to the constant.js

Test plan (required)

yarn start click on language select menu and pick Dutch

Final Checklist

• [x] Have you bumped the version in package.json?
  • Second decimal for major change, third decimal for minor change. This can go past 10 i.e. 1.0.9 !=> 1.1.0, 1.0.9 => 1.0.10
  • Read here for more info
• [x] Have you added any new tests necessary?
• [x] Is your PR rebased off the most current master?
• [x] Have you squashed all commits, or if you will merge will you squash all commits?
• [x] Did you use yarn, not npm?
• [x] Did you use Material-UI wherever possible?
• [x] Did you format according to Prettier?
• [x] Did you run all of your most recent changes locally to make sure everything is working?