Add Dutch language

[Jelcoo]

Added Dutch translations for Nginx Proxy Manager.
I hope everything was done correctly. If not, please let me know.

[nginxproxymanagerci]

CI Error:

/bin/bash: warning: setlocale: LC_ALL: cannot change locale (en_US.UTF-8)
certbot-node: Pulling from nginxproxymanager/nginx-full
Digest: sha256:34a4867931ac3ef308cd244202fc774edc306d201c8374839d6dee7fbe1d6b57
Status: Image is up to date for nginxproxymanager/nginx-full:certbot-node
docker.io/nginxproxymanager/nginx-full:certbot-node
�[1;34m❯ �[1;36mBuilding Frontend ...�[0m
yarn install v1.22.22
[1/4] Resolving packages...
[2/4] Fetching packages...
[3/4] Linking dependencies...
warning " > @uiw/react-textarea-code-editor@3.1.1" has unmet peer dependency "@babel/runtime@>=7.10.0".
[4/4] Building fresh packages...
Done in 40.98s.
yarn run v1.22.22
$ biome lint
src/locale/src/HelpDoc/index.ts:4:13 lint/correctness/noUnusedImports  FIXABLE  ━━━━━━━━━━━━━━━━━━━━

  ! This import is unused.
  
    2 │ import * as en from "./en/index";
    3 │ import * as ja from "./ja/index";
  > 4 │ import * as pl from "./pl/index";
      │             ^^
    5 │ import * as ru from "./ru/index";
    6 │ import * as sk from "./sk/index";
  
  i Unused imports might be the result of an incomplete refactoring.
  
  i Unsafe fix: Remove the unused imports.
  
     2  2 │   import * as en from "./en/index";
     3  3 │   import * as ja from "./ja/index";
     4    │ - import·*·as·pl·from·"./pl/index";
     5  4 │   import * as ru from "./ru/index";
     6  5 │   import * as sk from "./sk/index";
  

src/locale/src/HelpDoc/index.ts:8:13 lint/suspicious/noRedeclare ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  × Shouldn't redeclare 'pl'. Consider to delete it or rename it.
  
     6 │ import * as sk from "./sk/index";
     7 │ import * as zh from "./zh/index";
   > 8 │ import * as pl from "./pl/index";
       │             ^^
     9 │ import * as nl from "./nl/index";
    10 │ import * as it from "./it/index";
  
  i 'pl' is defined here:
  
    2 │ import * as en from "./en/index";
    3 │ import * as ja from "./ja/index";
  > 4 │ import * as pl from "./pl/index";
      │             ^^
    5 │ import * as ru from "./ru/index";
    6 │ import * as sk from "./sk/index";
  

Checked 213 files in 51ms. No fixes applied.
Found 1 error.
Found 1 warning.
lint ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  × Some errors were emitted while running checks.
  

info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
error Command failed with exit code 1.

[jc21]

I've resolved the issues in #4955 and merged those commits in. Thanks!