Starting from https://git.libreoffice.org/core/+/40bb9ac690d979ef544d5aa759bd734a176912a0, setting a language to a frame in slide master, and creating a new slide based on that master, gives a frame with initial language.
1. Create a new blank presentation
2. Open Master Slides sidebar; right-click on the only one master displayed in "Used in This Presentation" (Default), and choose "Edit Master..."
3. In the opened master, right-click at the border of the main frame (where a sample outline is displayed), and choose "Character..."
4. In Character dialog, open Font tab, and choose Language to anything different (if Arabic/Asian scripts are enabled, it's enough to only change Western script language) -> OK
5. Menu View->Normal
6. Menu Slide->New Slide (Ctrl+M)
7. Check the language of the frame of the newly created slide (using procedure from steps 3-4)
Expected: the language should be same as set at step 4
Actual: the language is same as before step 4
Tested with Version: 22.214.171.124 (x64)
Build ID: 98b30e735bda24bc04ab42594c85f7fd8be07b9c
CPU threads: 12; OS: Windows 10.0; UI render: GL; VCL: win;
Locale: ru-RU (ru_RU); UI-Language: en-US
and current master toward 6.4
Build ID: c9336bfb6bbf6d73d3f23c124262ade30133448d
CPU threads: 4; OS: Linux 4.15; UI render: default; VCL: gtk3;
Locale: ca-ES (ca_ES.UTF-8); UI-Language: en-US
Adding Cc: to Gülşah Köse