docs: document that we need to translate user-facing texts

[SteveLauC]

What does this PR do

1. Document that newly-added user facing texts need to be translated.
2. Add a check list item for it.

Standards checklist

• [x] The PR title is descriptive.
• [x] I have read CONTRIBUTING.md
• [ ] Optional: I have tested the code myself

For new steps

• [ ] Optional: Topgrade skips this step where needed
• [ ] Optional: The --dry-run option works with this step
• [ ] Optional: The --yes option works with this step if it is supported by the underlying command

If you developed a feature or a bug fix for someone else and you do not have the means to test it, please tag this person here.