Interlanguage links are links from a page in one Wikipedia language to an equivalent page in another language. These links can appear as links in the "Languages" section of the page's left sidebar.
Less commonly, for certain situations these can appear as inline links in the text of the article.
These two types of links are created and handled differently.
Links in the sidebar
A list of available languages is visible in the left sidebar of the desktop version of Wikipedia under the "Languages" section.
Pages on Wikipedia can link to equivalent pages in other languages. The English article on Spain includes a link to the Spanish article España in the sidebar, and vice versa. These links can be maintained in two places: in Wikidata and by using local links in the text of the page itself. Using Wikidata is the preferred method.
Click on Edit links to add links to the Wikidata page
Wikidata is a sister project of Wikipedia, it is a collaboratively edited knowledge base. Part of this project is to centralize the interlanguage links for all the Wikipedia projects. The Wikidata entry for a page in the left sidebar contains (among other things) a list of links for that page in different languages and it is the standard procedure to add interlanguage links.
Adding a new link
"Languages" in the sidebar shows the Wikidata language list. At the bottom is a link labelled "Edit links" (mobile viewers may lack such a feature). Clicking on this will reveal the Wikidata page:
Scroll down to the box marked "Wikipedia" and click on "edit" at the top of that box.
Click on the greyed-out "wiki" field, the final item on the list, and enter a language code (see list of ISO 639-1 codes for all language codes).
Click on the greyed-out "page" field (now visible) and type in the name of the article as it appears in that language's Wikipedia.
Click on "publish" at the top of the box.
If you get an error message like "Site link ... is already used by item Q ...", this means that there is already an item in Wikidata pointing to the article you are trying to link to. Wikidata does not allow more than one item to point to a single article in a Wikipedia. There are then two cases:
The two items really are the same, and should be merged: see d:Help:Merge.
The scope of the two items is not in fact the same (typically, the article in one language is wider in scope than the article in the other language) and they should not be linked, unless there exists a redirect between the two items on one of the languages. Unfortunately, there is no mechanism in Wikidata at present for linking articles that are related, but different in a scope – it is possible to do this with local links, however.
It may take several minutes for the link to show up in the article's sidebar. Especially, to enable links between templates, you must explicitly purge the server's cache for each template.
Finding interlanguage links for an article
There are many ways Wikipedia articles of another language can be found. These include translating the article's title and its synonyms to other languages and searching for them on the respective language-versions of Wikipedia or on Google, searching for articletitle site:wikipedia.org -site:en.wikipedia.org (for non-English Wikipedia articles), checking the other language versions of related or superordinate articles and checking the other language versions of the article's categories.
Editing a link
To edit an entry on Wikidata, click on the edit link next to the entry you wish to edit. A textbox will appear allowing you to edit the entry. Click on save when you are done.
Removing a link
You can remove a page from the list of Wikidata's links. To do so click edit, then click on the remove link.
Before 2013, the only method of generating a link in the sidebar to the article in another language was to use a "local" interlanguage link in the text of the page itself. As of 2013, the use of such "local links" for interlanguage linking has been deprecated, with interlanguage link data being centralized on Wikidata. The problem with this approach was that each language had to maintain its own separate lists. So for example, if the name of a page on the English Wikipedia changed, then each language that linked to that page would have to separately notice this fact and then change their own links.
For most pages, these links are no longer needed and can be safely removed, but you should verify that the local list and the Wikidata lists match before doing so. There is an automated tool to help with that.
The local links do however still serve some purposes:
They override the information that comes from Wikidata.
They are required if more than one article (or redirect) in one language should point to a target article in another language.
They are required in order to explicitly link to or from redirects. This can be used to level semantical or organizational differences between articles in different Wikipedias so that the links can go to the exact equivalent term in another language rather than to an article with a somewhat different scope or only related topic.
They are required in order to link to sections of articles. For example, corresponding to the English article spot welding, in Italian there is only a section rather than a full article, so spot welding includes the local link [[it:Saldatura#Saldatura a punti]].
They are required for pages that are not allowed on Wikidata, such as your userpage.
Although generally no longer required, to force the generation of a "local" interlanguage link in the Language sidebar of the article, use the following form:
NOTE: These links are treated specially, and don't show up in the body of the text, but in a special sidebar section "Languages" listed by language name. They can go anywhere in the article source as their placement does not alter the visual appearance of the links on the rendered page except for the order. However, the convention is to put them at the bottom of the page. Remember, you only need these type links if you are overriding the information from Wikidata, otherwise you should edit the links on the Wikidata entry.
Featured articles and good articles
Featured or good articles are represented via Wikidata.
When the English Wikipedia doesn't have an article, the links to it show up as red links. For example, we don't have an article on Erich Anders (it shows up as a red link), but the German Wikipedia does. It is possible to:
Best practice is to use the template ((interlanguage link)) which gives both a redlinked English link and a German blue link, but hides the German link if the English redlink turns blue when the article is created. For instance, by writing ((interlanguage link|Erich Anders|de)) we get "Erich Anders [de]", while by writing ((interlanguage link|Ludwig van Beethoven|de)) we get "Ludwig van Beethoven", as the page does exist on the English Wikipedia. (Alternatively, the shortcut ((ill)) may be used in place of ((interlanguage link)).)
If the foreign language article has a different title, use the following parameters: ((ill|English article title|language code|Title in foreign language)) – for example, ((ill|Marivan and Sarvabad constituency|fa|حوزه_انتخابیه_مریوان_و_سروآباد)) gives Marivan and Sarvabad constituency [fa].
To show the language code and the link to the other language article, but don't link to an English article: ''[[:fr:Jeux olympiques]]'' or [[:ja:東京都]] These links look like this: fr:Jeux olympiques or ja:東京都. The language abbreviation is displayed, so the reader knows that they are links to the French article on the Olympic Games and the Japanese article on Tokyo.
To show the name of the other language instead of its code, which can be easier to read for less experienced readers, you can use the lang template or one of its class: ((lang-fr|[[:fr:Jeux olympiques|Jeux olympiques]])) or ((lang-ja|[[:ja:東京都|東京都]])) These links look like this: French: Jeux olympiques or Japanese: 東京都.
To show only the title: ''[[:fr:Jeux olympiques|Jeux olympiques]]'' or [[:ja:東京都|東京都]] which would look like this in your text: Jeux olympiques or 東京都.
To link to a namespace other than the main article namespace, add the namespace after the language abbreviation. For example, the page for general discussions on the Japanese Wikipedia is located in the Wikipedia namespace. You can link to it as follows: [[:jp:Wikipedia:井戸端]] or [[:jp:Wikipedia:井戸端|]] giving jp:Wikipedia:井戸端 or Wikipedia:井戸端.
On Talk pages and on Meta, you can omit the leading colon. An interlanguage link like [[fr:Jeux olympiques]] will appear inline in the text, like a regular link.
An inline link to a Wiktionary entry, such as [[wikt:model|]], can be directed to another language's Wiktionary: ''[[wikt:fr:modèle|]]'' gives fr:modèle.
Links to pages that do not exist
On the English Wikipedia, links to pages that do not exist are displayed in a different color (usually red). But this check is not performed for links to pages on another wiki (including other Wikimedia Foundation sites). Unlike internal links, these links do not indicate whether the target page exists or not. If the target page does not exist, the link leads to a message on the target wiki informing you that the page does not exist and inviting you to create it.
If you find interlanguage links to non-existent pages on the English Wikipedia, they may be deleted as having no content. Alternatively, you can create at least a stub page on the target wiki before adding an interlanguage link to it. Later, interested people can complete the rest of the page.
Note: if the language prefix is wrong, it is considered to be part of the name of a page on the English Wikipedia. This page will probably not exist, so the link will be displayed in red.
You can also link to Wikipedia pages in other languages by using external links, in the same way as linking to other sites on the Internet. This method should not be used in the body of an article, but it may be useful on User pages or Talk pages. For example, this method can be used to link to diff pages that show the editing history of a page on another Wikipedia.
The following code:
Tokyo 2020 was added to the French page on the Olympics in [//fr.wikipedia.org/w/index.php?title=Jeux_olympiques&diff=96480208&oldid=96417677 this diff].
Tokyo 2020 was added to the French page on the Olympics in this diff.