You can use variable font for some writing system after Android Oreo.

In Android Oreo, it is the first time that Android support variable font. And Android add NotoSansKhmer-VF.ttf for Khmer language.

Line 214 in fonts.xml of Android Oreo

<!-- ... -->

<family lang="und-Khmr" variant="elegant">
    <font weight="100" style="normal">NotoSansKhmer-VF.ttf
        <axis tag="wdth" stylevalue="100.0" />
        <axis tag="wght" stylevalue="26.0" />
    </font>
    <font weight="200" style="normal">NotoSansKhmer-VF.ttf
        <axis tag="wdth" stylevalue="100.0" />
        <axis tag="wght" stylevalue="39.0" />
    </font>
    <font weight="300" style="normal">NotoSansKhmer-VF.ttf
        <axis tag="wdth" stylevalue="100.0" />
        <axis tag="wght" stylevalue="58.0" />
    </font>
    <font weight="400" style="normal">NotoSansKhmer-VF.ttf
        <axis tag="wdth" stylevalue="100.0" />
        <axis tag="wght" stylevalue="90.0" />
    </font>
    <font weight="500" style="normal">NotoSansKhmer-VF.ttf
        <axis tag="wdth" stylevalue="100.0" />
        <axis tag="wght" stylevalue="108.0" />
    </font>
    <font weight="600" style="normal">NotoSansKhmer-VF.ttf
        <axis tag="wdth" stylevalue="100.0" />
        <axis tag="wght" stylevalue="128.0" />
    </font>
    <font weight="700" style="normal">NotoSansKhmer-VF.ttf
        <axis tag="wdth" stylevalue="100.0" />
        <axis tag="wght" stylevalue="151.0" />
    </font>
    <font weight="800" style="normal">NotoSansKhmer-VF.ttf
        <axis tag="wdth" stylevalue="100.0" />
        <axis tag="wght" stylevalue="169.0" />
    </font>
    <font weight="900" style="normal">NotoSansKhmer-VF.ttf
        <axis tag="wdth" stylevalue="100.0" />
        <axis tag="wght" stylevalue="190.0" />
    </font>
</family>

<!-- ... -->

Android will use different font in different language automatically. You can directly set fontVariationSettings for Khmer language.

How to use variable font?

In this example, we set width to 25, and weight to 32.

<EditText
    <!-- ... -->
    <!-- Khmer language as example -->
    android:text="ភាសាខ្មែរ"
    android:fontVariationSettings="'wdth' 25, 'wght' 32" />

You should follow the syntax in Android document to fontVariationSettings.

…and check OpenType document to learn more about axis tags:

Which language have built-in variable font?

You can directly check fonts.xml. Switch branch for different version of Android. Search axis(tags in fonts.xml for variable font) in fonts.xml.

For example, this is a serif (fallbackFor="serif") font of Armenian (lang="und-Armn") that support variable. At regular weight (weight="400") Android will set its ‘wght’ to 400.

<family lang="und-Armn">
  <!-- ... -->

  <font weight="400" style="normal" fallbackFor="serif"
        postScriptName="NotoSerifEthiopic-Regular">NotoSerifEthiopic-VF.ttf
      <axis tag="wght" stylevalue="400"/>
  </font>

  <!-- ... -->
</family>

For English

Android add variable version of Roboto (default font of English) since Android 12. But for some reason, they use static version in regular weight.

<font weight="400" style="normal">RobotoStatic-Regular.ttf</font>

You can set textStyle to bold (or any other you want) to use variable version of Roboto. Then you can set any “text style” you like ("'wght' 400" for regular if you wish) by fontVariationSettings.

<EditText
    <!-- ... -->
    android:text="A quick brown fox jumps over a lazy dog"
    <!-- or any text style you want -->
    android:textStyle="bold"
    android:fontVariationSettings="'ital' 0.5, 'wdth' 85.5, 'wght' 450" />

For CJK (Chinese, Japanese and Korean)

Unfortunately, Android doesn’t include variable Noto CJK yet. There is a issue to request Google add variable version of Noto CJK into Android. Please star it to support the request.

My app

I wrote a app that use Android API to set variation. And it allows user to import own fonts to try.

App Preview en

WordlessEcho/Variable-Font-Test: A simple app to test variable font in system.