Translations

Basics

Everything starts with the various .xml files in translation/source. They define all the individual phrases that can be translated. Each file contains phrases for a certain concept or part of Lichess, e.g. swiss.xml for swiss tournaments, faq.xml for the FAQ, etc. site.xml is the main file that contains all site-wide phrases and everything that doesn't fit anywhere else.

Each phrase has a key/name and the original text in British English, e.g.

<string name="playWithAFriend">Play with a friend</string>

They can have placeholders marked by %s. These can be replaced by something, i.e. the number of games or a separate translated string that needs to be formatted differently or be a clickable link. %1$s, %2$s, etc. can be used for multiple placeholders.

<string name="xStartedStreaming">%s started streaming</string>
<string name="xStartedFollowingY">%1$s started following %2$s</string>

There are also plurals elements for phrases that need to change depending on the value of a placeholder:

<plurals name="nbBlunders">
  <item quantity="one">%s blunder</item>
  <item quantity="other">%s blunders</item>
</plurals>

The content of those .xml files are automatically uploaded to crowdin.com/project/lichess where volunteers translate them. The resulting translations are again automatically downloaded and regularly merged in PRs named "New Crowdin updates", resulting in another batch of .xml files in translation/dest/<area>/<lang-code>.xml where <area> is the faq, site, etc. from above.

Using translations in Scala

There's a Scala equivalent for every translation key defined in modules/i18n/src/main/I18nKeys.scala. This file can be generated automatically from the source .xml files by running bin/trans-dump.

It defines an I18nKeys object with values for each translation key:

object I18nKeys {
    val `playWithAFriend` = new I18nKey("playWithAFriend")
    // ...

    object swiss {
        val `swissTournaments` = new I18nKey("swiss:swissTournaments")
        // ...
    }
}

That object is generally imported with import lila.i18n.{ I18nKeys => trans } so that keys can be accessed as trans.theKey for values from site.xml. Other areas need to be specified, e.g. trans.swiss.theKey for swiss.xml, etc.

Each key is an instance of I18nKey defined in modules/i18n/src/main/I18nKey.scala which defines everything that can be done with them:

final class I18nKey(val key: String) {
  def apply(args: Any*)(implicit lang: Lang): RawFrag = // ...
  def plural(count: Count, args: Any*)(implicit lang: Lang): RawFrag = // ...
  def pluralSame(count: Int)(implicit lang: Lang): RawFrag = plural(count, count)

  def txt(args: Any*)(implicit lang: Lang): String = // ...
  def pluralTxt(count: Count, args: Any*)(implicit lang: Lang): String = // ...
  def pluralSameTxt(count: Int)(implicit lang: Lang): String = pluralTxt(count, count)
}

There are three different functions, each in two versions, one producing a fragment, i.e. for use in scalatags when generating HTML, and another producing a String.

Applying a key directly as trans.theKey() directly gives a translated fragment. Values to replace placeholders can be passed directly as arguments: trans.theKey("abc", 42).
To use plural keys you need to use trans.theKey.plural(42, "abc", 42). The first number determines the plural version to use (i.e. "game" vs "games") and everything after that is replacing the placeholders. Note that the initial value has to be repeated in the correct position.
For the common case when there is only one value that both determines the plural and needs to be inserted, there is trans.theKey.pluralSame(42).

And then there are the three String version equivalents trans.theKey.txt, trans.theKey.pluralTxt and trans.theKey.pluralSameTxt.

What is `implicit Lang`?

All these functions have an implicit Lang parameter that specifies the target language to translate to when they are called. This parameter can be automatically extracted from an implicit Context in scope. Generally, that means adding additional (implicit ctx: Context) or (implicit lang: Lang) argument lists to all functions calling one of the translation functions, all the way up the call stack until the controller endpoints which will have an explicit Context available.

Using translations in JavaScript

For JS, the Scala code generally passes a JSON object with { transKey: value } down to the JS controller for each page. Often, a list of keys to send down is defined in app/views/<area>/bits.scala which is then converted to JSON using the i18nJsObject function brought into scope by an import lila.app.templating.Environment._ (and originating in app\templating\I18hHelper.scala).

Here's an example from the swiss tournament pages: app/views/swiss.bits.scala

import lila.app.templating.Environment._
import lila.i18n.{ I18nKeys => trans }

// ...

def jsI18n(implicit ctx: Context) = i18nJsObject(i18nKeys)

private val i18nKeys = List(
  trans.join,
  trans.withdraw,
  trans.youArePlaying,
  // ...
  trans.swiss.viewAllXRounds,
  // ...
).map(_.key)

This object is passed to the call initializing the JS controller: app/views/swiss/show.scala

embedJsUnsafeLoadThen(s"""LichessSwiss.start(${safeJsonValue(
  Json.obj(
    "data"   -> data,
    "i18n"   -> bits.jsI18n,
    // ...
)})""")

The JS controller then uses this JSON object to construct a Trans object: ui/swiss/src/ctrl.ts

this.trans = lichess.trans(opts.i18n);

The lichess.trans function is defined in ui/site/src/component/trans.ts.

The `Trans` object

The resulting Trans object provides similar functions as I18nKey in Scala:

trans("theKey", 42, "abc") will give the translation for "theKey" as a string, replacing placeholders with 42 and "abc".
trans.noarg("theKey") is a fast-path for keys without placeholders
trans.plural("theKey", 42, 42, "a", "b", "c") is for plural forms and the same as pluralTxt in Scala
The first argument is the translation key
The second argument is the number used to decide the plural form
The subsequent arguments are used as replacements for the placeholders
trans.vdom("theKey", h("a", ...)) can be used when the placeholders are vdom nodes and returns a list of strings and nodes.
trans.vdomPlural("theKey", 42, h("a", 42)) is the same for plural forms