Use tags with spaces and numbers in toml?

I would like to have a tag named 1 to 18 days in Hugo:

---
date: 2007-01-01T00:00:35Z
tags:
  - 1 to 18 days
---
... post content ...

When I compile with hugo --i18n-warnings I get the following warnings: i18n|MISSING_TRANSLATION|en|1 to 18 days.

When I want to add the translation to /i18n/en.toml or /i18n/fr.toml it does not work since the tag begins with a number and contains spaces.

This is invalid in Toml in /i18n/en.toml:

[1 to 18 days]
other = "1 to 18 days"

Here is the stackoverflow question.