Does the National Library have any English-language texts of party conference speeches by Plaid Cymru leaders from the last few decades?
Ateb
The Welsh Political Archive at the National Library does hold a most substantial archive of material deposited by Plaid Cymru. This includes (within Class G) records relating to the party's annual conferences.
However, these include mainly the minutes of the proceedings at the various conferences, correspondence, and papers deriving from the arrangements for the conferences.
For more information regarding the Welsh Political Archive, you may follow the link provided:
http://www.llgc.org.uk/index.php?id=503
The best source concerning speeches delivered at the conferences would be the reports published in the newspapers, but these would simply provide summaries of the contents of these speeches.
To search for more information for yourself, you may follow the link provided to search the National Library's full online catalogue:
https://darganfod.llyfrgell.cymru/primo-explore/search?vid=44WHELF_NLW_NUI&lang=en_US