Remove translatable content from product config

[ghost]

After the initial localization run (yay, now), having the localization in the normal config files is an issue from a few angles:

• The strings are painful to extract/reintegrate.
• For the source format, it is decidedly not ideal to keep all translations in the same file, it makes a lot more sense to separate by language, so the individual files can be spread out for translation by different people.
• Reusing translation strings is hard currently (have not tried entities but that is essentially the only real option for reuse currently).
• There is no good way of getting an overview of strings that still need to be translated.
• We already have a translation format for fragments, that works quite well. It would make sense to reuse that (or even use the same files that we also use for fragments, so we can share strings between those two components).

Ultimately, it might actually make sense to handle the product config translations entirely as part of simplify.js, so we don't need to change anything about the build-navigation process.