Wikipedia:Interwiki
Interwikis (short for interwikimedia links) are links between wikis. These often show links to the same page in other languages. They can also be used to link to other projects, such as Wiktionary and Wikimedia Commons.
It is important to have interwikis to other languages. They help people to find more information and other Wikipedias. Most articles on the Simple English Wikipedia have pages in the English Wikipedia or other languages.
Links in the sidebar to other languages change
The way to add interwiki links to other languages is done using Wikidata. This allows you to link a page to the same page in all the other languages. These links appear in the left sidebar, under the "languages" heading. The link to Wikidata also enables other things, like Authority control.
Adding a new link change
To add a new link, look to the bottom of the language heading in the left sidebar. You will find a link called Change links. Click on it to open a window that allows you to create the link. Once you add a link to one language, the Wikidata connection is made. This will cause all of the other linked languages to appear in the list as well.
Local links change
Before wikidata, these links were made in the text of the page itself. The issue with this is that new language links were not added automatically. And each language had to maintain their own list of links. If you see these, you can safely remove them if Wikidata is connected.
Non-sidebar links to other language Wikipedias change
You may at times have the need to link directly to a different page in another language Wikipedia. In other words, a page on another wiki but not another language of the same page. These type of links should be kept to a minimum.
Adding a new link change
You can create these links just like a normal wikilink. Simply add the two-letter language code to the beginning with a colon (:) at the front. For example, if you are linking to the English Wikipedia's Internet article, use the following code:
[[:en:Internet]]
Linking to other Wikimedia projects change
First of all, check Wikipedia:Sister projects to see if a template in your article would work for what you want to do.
If not, find where you want the link in the article, and type the code for the project and the page title in that project in square brackets. You could also type text to appear instead of the link. For a link to the main page of the Simple English Wiktionary, you would type:
[[wiktionary:simple:Main Page|Simple English Wiktionary]]
'simple:' tells Wiktionary what language to display itself in. The link looks like this:
For projects in the same language, just the project interwiki is needed. For example:
[[wikt:Main Page|Main Page of Simple English Wiktionary]]
Wiki look like this: