Open Settings > Time & Language > Language > Add a Preferred Language.
Type “Japanese” and select it from the list.
Select “Install Language Pack,” choose any or all of the features you would like to install (such as text-to-speech and handwriting) and click Install. Do not choose “Set as my display language” unless you want Windows to be in Japanese as well.
Once the Japanese Language Pack has finished installing, you can hit the Windows Key + Space Bar to toggle languages from your default language to Japanese. In the bottom right corner of your screen, you will either see the letter A or the hiragana あ. Click this button to toggle between romaji input or kana/kanji input. When the あ is showing, your input will be in kana.