Add example startup action to switch to browser language

editions/full/tiddlers/Startup-SetLanguage.tid

@@ -0,0 +1,4 @@
+title: $:/_Startup/SetLanguage
+tags: $:/tags/StartupAction
+
+<$action-setfield $tiddler="$:/language" text={{{ [[$:/languages/en-GB]] [plugin-type[language]sort[description]removeprefix[$:/languages/]] +[prefix{$:/info/browser/language}] ~[[en-GB]] +[addprefix[$:/languages/]] }}}/>

editions/tw5.com/tiddlers/features/StartupActions.tid

@@ -1,10 +1,10 @@
 created: 20180323092308399
-modified: 20180323092547312
+modified: 20190307183936035
 tags: Features
 title: StartupActions
 type: text/vnd.tiddlywiki
 
-During startup, TiddlyWiki executes any ActionWidgets found in tiddlers with the following system tags:
+TiddlyWiki executes any ActionWidgets found in tiddlers with the following system tags during startup:
 
 * <<tag "$:/tags/StartupAction">> (executed on all platforms)
 * <<tag "$:/tags/StartupAction/Browser">> (only executed when running in the browser)
@@ -22,6 +22,12 @@ Startup actions are useful for customising TiddlyWiki according to environmental
 </$reveal>
 ```
 
+<<.from-version "5.1.20">> This example switches to the language plugin matching the language reported by the browser:
+
+```
+<$action-setfield $tiddler="$:/language" text={{{ [[$:/languages/en-GB]] [plugin-type[language]sort[description]removeprefix[$:/languages/]] +[prefix{$:/info/browser/language}] ~[[en-GB]] +[addprefix[$:/languages/]] }}}/>
+```
+
 Note that global macros are not available within startup action tiddlers by default. If you need to access them then you'll need to explicitly include them with an ''import'' [[pragma|Pragma]] at the top of the tiddler:
 
 ```