As of July 2018, it is possible to translate Hiri into your language. There are three different ways you can translate Hiri:
The simplest and recommended approach is just to go to Hiri project on Weblate and start translating. This way you can translate simultaneously with other people and the translations will automatically propagate to the main Hiri project. If you don't want to open an account on Webplate you won't be able to save translations but you can still make suggestions.
If you don't like the Webplate UI you can use Qt Linguist desktop application. Qt Linguist is distributed only as a part of Qt framework which is huge (a couple of GB). Luckily there is a GitHub project which has the standalone releases which you can download here. In order to work with with Qt Linguist you need to have the translation file available locally on your computer.
Further instructions on Github
Any text editor
You don't have to use Qt Linguist either. Qt translation files are human readable XML files, so you can edit them in any text editor. Then you can upload them to Webplate or commit changes with git. Just be careful that you properly quote special characters inside translation tags and to update type attribute from unfinished to finished.
Please send an email to firstname.lastname@example.org