Add first Dutch translation

[studio1902]

This is the first Dutch translation. I translated it without using v3 IRL but it should be a pretty solid basis. Will do additional PR's with bug fixes when I start using V3 more in the coming weeks.

[ebeauchamps]

In nl.json, you will have to manually add (and translate) the following keys:
Create a Form
Create a Global Set
Create a Role
Create a Taxonomy
Create a User Group

[ebeauchamps]

In fieldtypes.php, you will have to manually add and translate:
textarea.config.character_limit
relationship.config.mode

You will have to check those fieldtypes in the English version of the CP to get the sentences that are to be translated.

[studio1902]

Thanks @ebeauchamps. Fixed it. I’ve encountered some error’s with the CLI Translator tool at the end for one of the reviews. I think it was for fieldtypes.php. That might explain why those strings were missing.

[plompd]

A couple of suggestions

[ebeauchamps]

@studio1902 An issue regarding my above comments was first reported in #1640

Missing keys were also reported in #1697 but you should check for the great work by @Konafets in #1904
Once his PR is merged, we should be able to get more keys to translate.

By the way, great work of yours here!