Some_Test.cls

@isTest
public with sharing class Some_Test {
    private static final Map<String, String> lang2key = new Map<String, String>{'English' => 'en_US', 'Japanese' => 'ja', 'Simplefied_Chinese' => 'zh_CN'}; 

    private static User setLang(String lang) {
        User me = [SELECT Id, LanguageLocaleKey FROM USER WHERE Id = :UserInfo.getUserId() LIMIT 1];
        me.LanguageLocaleKey = lang2key.get(lang);
        update me;
        return me;
    }

    @isTest static void testGetSomeLabel() {
        Test.startTest();

        System.runAs(Some_Test.setLang('English')) {
            // getSomeLabel(String key) returns key's label.
            String someLabel = Some.getSomeLabel('key');
            System.assertEquals('Key', someLabel, 'label is not correct');
        }

        System.runAs(Some_Test.setLang('Japanese')) {
            // getSomeLabel(String key) returns key's label.
            String someLabel = Some.getSomeLabel('key');
            System.assertEquals('キー', someLabel, 'ラベルが正しくありません');
        }

        Test.stopTest();
    }
}