Initial multilanguage support

[mirober]

These changes should allow at least a basic form of multi language support. An uncompleted example for Dutch (nld) is provided

Things that need to be done to support a new language:

• Change language parameter in settings.toml to the tag of the new language
• Create a new folder in mathfly.config with the tag as the name, any configuration file copied into this folder will be imported instead of the English files.
• Create a new integer content file in mathfly.lib.numbers, either containing a reference to one of the dragonfly implementations or a new one.

Perhaps in the future we will just grab the current language from Dragon instead of requiring the settings change, but I think for now this is simpler for testing.