Try adding some window specific commands

autoload/langserver/api/window.vim

@@ -0,0 +1,2 @@
+function! langserver#api#window#showMessage()
+endfunction

autoload/langserver/window.vim

@@ -0,0 +1,30 @@
+let s:message_type = {
+      \ 'name': {
+        \ 'error': 1,
+        \ 'warning': 2,
+        \ 'info': 3,
+        \ 'log': 4,
+        \ },
+      \ 'number': {
+        \ 1: 'error',
+        \ 2: 'warning',
+        \ 3: 'info',
+        \ 4: 'log',
+        \ },
+      \ }
+
+function! langserver#window#handle#showMessage(message) abort
+  let l:type = a:message['type']
+  let l:message = a:message['message']
+
+  echo l:type | echo l:message
+endfunction
+
+function! langserver#window#handle#logMessage(message) abort
+  let l:type = a:message['type']
+  let l:message = a:message['message']
+
+  " Not sure if this will work exactly.
+  " Might have to do another mapping here.
+  call langserver#log#log(l:type, l:message)
+endfunction