Use Crowdin for translations

We don't have any tooling for keeping track of which newly added english texts need to be translated into other languages. Nobody really knows how out of date our translations are, or how to find out. Instead of investing in such tooling ourselves, I suggest we look into using crowdin.com instead.

We recently started using Crowdin at work, and I was pleasantly surprised at what a nice and powerful platform this is. Basically you upload your english text file, and then contributors can swarm on typing in translations for various languages, all with a pretty nice web interface. There's a dashboard showing how many texts are left to be translated for each language, and also which texts have "quality issues" (an example being that the english text ends with a period, but the translation doesn't, and many others). And of course it can pre-translate things from Google Translate or DeepL automatically.

This requires that we have data files (json or yaml) instead of code for our translations, so we'd need to change our code a bit to serialize things to/from json or yaml. That doesn't sound very hard to me.

Crowdin is not cheap, but it has a [free plan](https://crowdin.com/page/open-source-project-setup-request) for open source projects which I think is very attractive.