User Defined Languages do not show up in the alphabetized Language > R or similar… they show up at the bottom of the Language menu, below the Language > User Defined Language > … submenu (for your v8.1.3; or, if you are in as old of a Notepad++ as Steen-Hultin’s UDL screenshots show, below the Language > Define Your Language…).
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… in my screenshot, ABC, Markdown (preinstalled), Markdown (preinstalled dark mode), and TI-Basic are all UDL that I can select.
The file UserDefineLangs.xml contains an entry for this UDL.
If that’s true, then Language > reStructuredText should show up near the Markdown entry, and Language > Define Your Language (or in a modern Notepad++, Language > User Defined Language > Define Your Language) should have reStructuredText in the User Language pulldown – see that mine shows the same languages in that pulldown that were in my menu screenshot:
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If you cannot figure it out from this, you might want to give us a screenshot of one or both of those locations to aid in debugging, along with going to the ?-menu Debug Info entry, clicking on the Copy debug info into clipboard hyperlink, and paste that into your reply as well.