Add Welsh translation state diagram

diagrams/state-diagrams/welsh-translation-states-and-fields.md

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+# Adding Welsh to a Form
+
+This document describes how adding a Welsh translation affect a form and it's form documents.
+
+## The Welsh tasklist status
+
+The Welsh task is optional. If the form creator does nothing, it will not block the form from being made live.
+
+If the task is started, by entering any values into the translation screen and submitting it as unfinished, the status will be set to in progress.
+
+The form cannot be made live while the Welsh task is in progress.
+
+If the task is completed, by entering a full translation of the form and submitting it as finished, the status will be set to complete and the form can be made live.
+
+Any changes to the form or translation which would mean that the Welsh is no longer complete, such as adding a declaration, new question, exit page or support method will 
+set the status to in progress.
+
+```mermaid
+stateDiagram-v2
+    Optional --> In_Progress:Enter Welsh, save unfinished
+    Optional --> Complete:Enter Welsh, save finished
+    Complete --> Optional:Delete Welsh
+    In_Progress --> Optional:Delete Welsh
+    In_Progress --> Complete:Enter Welsh, save unfinished
+    Complete --> In_Progress:Invalidate Welsh
+```
+
+## New fields on Form
+
+## available_languages
+
+`available_languages`, is an array of language codes. It's `["en"]` by default, which signals that the form is only available in English.
+Forms with a Welsh translation have `["en", "cy"]` in the array.
+
+As soon as the Welsh translation page is submitted with at least one value for a field, the `available_languages` array will be updated.
+
+The `available_languages` array is used when generating form documents, which are used by the runner. Form documents are created for each language in the array.
+It's updated when the Welsh translation is submitted so the user can immediately preview the Welsh translation, even if they haven't completed it.
+
+The `available_languages` array is saved in form documents. It's used by the runner to discover that the form is available in Welsh, if it's showing the English version.
+
+### welsh_completed
+`welsh_completed` is a boolean, which is records if the form creator has marked the Welsh as complete. It's not included in form_documents.
+
+## Places where the values change
+
+If the English form changes, we need to update the Welsh translation to match. This can happen if the form creator deletes a field, such as a declaration.
+To keep the Welsh translation in sync, Form has a new method
+`normalise_welsh!`. It's called when the form documents for draft or live are
+made. It ensures that the Welsh translation doesn't have any values which the
+English form doesn't have.
+
+The method on Form in turn calls `normalise_welsh!` on each page and condition, which syncs these values.