---
title: Determine languages
slug: cms-13/docs/determine-languages
docTags: 
createdAt: 2026-07-30T20:22:00.693Z
---

Optimizely Content Management System (CMS) uses three language setting types:

- **System language** – Controls date and time formatting, sort order, and number formatting.
- **User interface language** – Controls which translated resources display and sets the language of the editing UI.
- **Content language** – The preferred language for displaying content.

For details about the language management concept, go to [Globalization](docId\:JgUTS4f5zIXWl2Iuj0jKY).

## Determine system language

The system language controls date formatting, sort order, and other culture-dependent behavior. Determine the current value or override it at runtime.

Read the current value from `CultureInfo.CurrentCulture`. Set it through `Thread.CurrentThread.CurrentCulture`. Retrieve the language code (such as `en-GB` or `sv-SE`) from `CultureInfo.Name`.

:::hint{type="info"}
**Note**
`CurrentCulture` cannot be a neutral culture because it provides sorting and formatting information. The language code must be a specific culture such as `en-GB` or `sv-SE`. Assigning a neutral culture to `CurrentCulture` throws an exception.
:::

The following rules determine the system language:

- Outside the edit and admin UI, the system uses the content language.
- For a logged-in user with a preferred language, the system uses that selection.
- Use the setting from `GlobalizationSettingsOptions.CultureLanguageCode`. If the value is set to Auto, the browser language preferences apply.

## Determine user interface language

The UI language determines which translated text appears for interface elements. Override it at runtime to test localized resources.

Read the current value from `CultureInfo.CurrentUICulture`. Set it through `Thread.CurrentThread.CurrentUICulture`. Retrieve the language code (such as `en` or `sv`) from `CultureInfo.Name`.

`CurrentUICulture` accepts neutral cultures because the runtime does not use it for sorting or formatting. For example, `CultureInfo.GetCulture("en")` returns a valid culture for `CurrentUICulture`.

The following rules determine the UI language:

- Outside the edit and admin UI, the system uses the content language.
- For a logged-in user with a preferred language, the system uses that selection.
- Use the setting from `GlobalizationSettingsOptions.UICultureLanguageCode`. If the value is set to Auto, the browser language preferences apply.

## Determine content language

The content language controls which language version of content each visitor sees. Read or change the value at runtime to support language-specific logic.

Read the current value from `IContentLanguageAccessor.Language` or the static accessor `ContentLanguage.PreferredCulture`. Change the value by assigning this property. Content language accepts neutral cultures. When sorting or formatting requires a specific culture, call `CultureInfo.CreateSpecificCulture`. This method follows .NET rules for deriving a specific culture from a neutral one. See the Microsoft documentation for [`CultureInfo.CreateSpecificCulture`](https://docs.microsoft.com/en-us/dotnet/api/system.globalization.cultureinfo.createspecificculture?view=net-5.0).

The following rules determine the preferred content language:

- If the hostname is associated with a specific language, that language applies.
- If a language segment exists in the URL (for example, `http://company.com/en/info`), that language applies.
- If a language site is selected from **Sites** in the edit view, that language applies.
- If the request contains a query parameter or cookie named `epslanguage`, that language applies.
- If `GlobalizationSettingsOptions.UseBrowserLanguagePreferences` is `true`, the browser language preference applies.
- If a language is mapped to wildcard host `*`, that language applies.
- Use the value from `GlobalizationSettingsOptions.UICultureLanguageCode`.
- When no other rule matches, the system uses the first enabled language branch in **Admin** > **Language Branches**. This branch serves as the default language.

:::hint{type="info"}
**Note**
After the preferred content language is determined, a second step determines the actual language to display. If the content does not exist in the preferred language, a fallback process begins. For pages, the page language settings in the edit view define the fallback. Go to [Globalization scenarios](docId\:JgUTS4f5zIXWl2Iuj0jKY) for details about language-specific content settings.
:::

## Strict language routing

Strict language routing returns an error when the URL and language-host mapping do not match. Disable or adjust this behavior to support more flexible URL patterns.

The following examples show how language routing works. A website has a page at root named `News` in English and `Nyheter` in Swedish. With no language mapping defined for site hosts, URLs resolve as follows:

- `http://localhost/News/` – Returns 404 because a language segment is required when no language-host mapping is configured.
- `http://localhost/en/News/` – Displays the page in English.
- `http://localhost/sv/Nyheter/` – Displays the page in Swedish.
- `http://localhost/Nyheter/` – Returns 404 (same reason as the first example).
- `http://localhost/en/Nyheter/` – Returns 404 because the URL segment language does not match the language segment.

When a language-to-host mapping is defined, `http://localhost/en/News/` returns a 404. The language segment must not appear in the URL when a host mapping defines a language.

Set `RoutingOptions.StrictLanguageRouting` to `false` to relax strict language routing.