I want to have my page numbers in LibreOffice Impress to be in Persian. Which means ۰۱۲۳۴۵۶۷۸۹ instead of 0123456789.
These settings work across all LibreOffice applications and documents, so you can make changes from any of the set of applications.
Changing the locale
You can set Persian as the default for all of your documents or for the current document only.
From the menu bar, Tools >> Options, then expand the Language Settings and click Languages
Change Locale to Farsi. (Note that this is independent of the setting for the user interface above.)
Change CTL to Default - Farsi. This will also automatically check Enabled for complex text layout (CTL) below. CTL handles right-to-left languages.
Check For the current document only according to your prefrence.
Changing the global numeral style
Click on Complex Text Layout on the left side of the dialog and change numerals to Hindi. (not shown in this screenshot)
Use Insert >> Page Number to show the Header/Footer dialog in order to add page numbers to your slides.
Setting numerals to Hindi in the Language Settings lets you use "Arabic" numbers without using an input method editor such as Ibus. In this context, of course, Arabic numerals are 1,2,3...
Is it possible to use Hindi numbers in the slide footer page number function without changing every number to Hindi?
Short answer: not through the GUI settings. Apart from the global number style setting, there does not appear to be any way to modify the function that inserts page numbers.
Interestingly enough, the footer lets you easily change the date to Persian. I believe the page numbers cannot be changed in a similar way because the page numbers appears to be drawn from functions to work with the printer.
The solution is to not use the built-in footer page numbering at all and instead insert your own text shape in the same area with the text set the way you want. For a small number of slides, this would not be too bad; but would not be feasible with more than a handful.
The solution to this problem is to use a LibreOffice Basic macro to insert the page numbers for you. For a short overview of LibreOffice macros and how to use them in your document, please see this answer.
Here is the macro code: