[docs] improve issue template for i18n
# What does this PR do?

The initial issue template (https://github.com/huggingface/transformers/pull/20199) includes minor typos and a closed PR link.
It makes it seem that the index page of a new language is already translated when it isn't.
Also some comment regarding what `langCode` or `langName` is would be helpful.

[x] Fixed typos.
[x] Removed irrelevant PR link.
[x] Explained what `langCode` or `langName` is more easily. (Replaced it rather)

<!-- Remove if not applicable -->

Fixes https://github.com/huggingface/transformers/issues/20955


## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?
May you please review this PR, @sgugger?
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20956). All of your documentation changes will be reflected on that endpoint.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.