The Localizing functions guide doesn't say how to add another language pack

[sequba]

Description

Pointed out by @kirszenbaum in https://github.com/handsontable/hyperformula/pull/1099#issuecomment-1317499945

Links

#1099

Type of issue

• [x] Missing documentation
• [ ] Error in the documentation
• [ ] Request for a demo
• [ ] Request for a tutorial, guide etc.
• [ ] Other issues, suggestions or ideas