feat(prompts): add configurable response language (#1246)

Closes #1086

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>

Author: deathbeam

README.md

@@ -171,14 +171,13 @@ The mappings can be customized by setting the `mappings` table in your configura
 - `normal`: Key for normal mode
 - `insert`: Key for insert mode
 
-For example, to change the submit prompt mapping or show_diff full diff option:
+For example, to change the complete mapping to Tab or show_diff full diff option:
 
 ```lua
 {
     mappings = {
-      submit_prompt = {
-        normal = '<Leader>s',
-        insert = '<C-s>'
+      complete = {
+        insert = '<Tab>'
       }
       show_diff = {
         full_diff = true
@@ -444,6 +443,7 @@ Below are all available configuration options with their default values:
   model = 'gpt-4.1', -- Default model to use, see ':CopilotChatModels' for available models (can be specified manually in prompt via $).
   tools = nil, -- Default tool or array of tools (or groups) to share with LLM (can be specified manually in prompt via @).
   sticky = nil, -- Default sticky prompt or array of sticky prompts to use at start of every new chat (can be specified manually in prompt via >).
+  language = 'English', -- Default language to use for answers
 
   resource_processing = false, -- Enable intelligent resource processing (skips unnecessary resources to save tokens)
 

lua/CopilotChat/config.lua

@@ -18,6 +18,7 @@
 ---@field model string?
 ---@field tools string|table<string>|nil
 ---@field sticky string|table<string>|nil
+---@field language string?
 ---@field resource_processing boolean?
 ---@field temperature number?
 ---@field headless boolean?
@@ -58,6 +59,7 @@ return {
   model = 'gpt-4.1', -- Default model to use, see ':CopilotChatModels' for available models (can be specified manually in prompt via $).
   tools = nil, -- Default tool or array of tools (or groups) to share with LLM (can be specified manually in prompt via @).
   sticky = nil, -- Default sticky prompt or array of sticky prompts to use at start of every new chat (can be specified manually in prompt via >).
+  language = 'English', -- Default language to use for answers
 
   resource_processing = false, -- Enable intelligent resource processing (skips unnecessary resources to save tokens)
 

lua/CopilotChat/config/prompts.lua

@@ -2,6 +2,7 @@ local COPILOT_BASE = [[
 When asked for your name, you must respond with "GitHub Copilot".
 Follow the user's requirements carefully & to the letter.
 Keep your answers short and impersonal.
+Always answer in {LANGUAGE} unless explicitly asked otherwise.
 <userEnvironment>
 The user works in editor called Neovim which has these core concepts:
 - Buffer: An in-memory text content that may be associated with a file

lua/CopilotChat/init.lua

@@ -473,6 +473,7 @@ function M.resolve_prompt(prompt, config)
 
   if config.system_prompt then
     config.system_prompt = config.system_prompt:gsub('{OS_NAME}', jit.os)
+    config.system_prompt = config.system_prompt:gsub('{LANGUAGE}', config.language)
     if state.source then
       config.system_prompt = config.system_prompt:gsub('{DIR}', state.source.cwd())
     end