Docs: Describe how to add a custom language pack

[sequba]

Context

Add section Creating a custom language pack to localizing-functions guide.

Types of changes

• Breaking change (a fix or a feature because of which an existing functionality doesn't work as expected anymore)
• New feature or improvement (a non-breaking change that adds functionality)
• Bug fix (a non-breaking change that fixes an issue)
• Additional language file, or a change to an existing language file (translations)
• Change to the documentation

Related issues:

1. Fixes The Localizing functions guide doesn't say how to add another language pack #1103

Checklist:

• I have reviewed the guidelines about Contributing to HyperFormula and I confirm that my code follows the code style of this project.
• I have signed the Contributor License Agreement.
• My change is compliant with the OpenDocument standard.
• My change is compatible with Microsoft Excel.
• My change is compatible with Google Sheets.
• I described my changes in the CHANGELOG.md file.
• My changes require a documentation update.
• My changes require a migration guide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.22%. Comparing base (31f70f9) to head (38e940e).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1551   +/-   ##
========================================
  Coverage    97.22%   97.22%           
========================================
  Files          169      169           
  Lines        14526    14526           
  Branches      3171     3171           
========================================
  Hits         14123    14123           
  Misses         396      396           
  Partials         7        7           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[github-actions]

Performance comparison of head (38e940e) vs base (31f70f9)

                                     testName |   base |   head | change
------------------------------------------------------------------------
                                      Sheet A | 516.29 | 518.43 | +0.41%
                                      Sheet B |    166 | 169.28 | +1.98%
                                      Sheet T | 147.12 | 149.97 | +1.94%
                                Column ranges |  512.4 | 519.65 | +1.41%
Sheet A:  change value, add/remove row/column |     14 |  13.97 | -0.21%
 Sheet B: change value, add/remove row/column | 128.15 |    134 | +4.56%
                   Column ranges - add column | 149.04 | 160.74 | +7.85%
                Column ranges - without batch |  431.8 | 458.31 | +6.14%
                        Column ranges - batch |  111.9 | 110.29 | -1.44%