Fork the repository, edit the appropriate files, send me a pull request
Locales are made by copying the messages.xlf file to a file named messages.[locale].xlf - eg, messages.jp.xlf
Within the file are a list of source entries, like so:
<trans-unit id="Welcome" datatype="html">
<source>Welcome</source>
<context-group purpose="location">
<context context-type="sourcefile">app/login-display/login-display.component.html</context>
<context context-type="linenumber">1</context>
</context-group>
</trans-unit>
Add an entry below the <source> tags called <target> and add the translation:
<trans-unit id="Welcome" datatype="html">
<source>Welcome</source>
<target>ようこそ</target>
<context-group purpose="location">
<context context-type="sourcefile">app/login-display/login-display.component.html</context>
<context context-type="linenumber">1</context>
</context-group>
</trans-unit>