CCM-9284 Letter language copy update (#416)

Author: nicki-nhs

frontend/src/__tests__/components/forms/LetterTemplateForm.test.tsx/__snapshots__/LetterTemplateForm.test.tsx.snap

@@ -225,7 +225,7 @@ exports[`Client-side validation triggers 1`] = `
               for="letterTemplateLanguage"
               id="letterTemplateLanguage--label"
             >
-              Additional language
+              Letter language
             </label>
             <div
               class="nhsuk-hint"
@@ -726,7 +726,7 @@ exports[`renders page one error 1`] = `
               for="letterTemplateLanguage"
               id="letterTemplateLanguage--label"
             >
-              Additional language
+              Letter language
             </label>
             <div
               class="nhsuk-hint"
@@ -1235,7 +1235,7 @@ exports[`renders page with multiple errors 1`] = `
               for="letterTemplateLanguage"
               id="letterTemplateLanguage--label"
             >
-              Additional language
+              Letter language
             </label>
             <div
               class="nhsuk-hint"
@@ -1689,7 +1689,7 @@ exports[`renders page with preloaded field values 1`] = `
               for="letterTemplateLanguage"
               id="letterTemplateLanguage--label"
             >
-              Additional language
+              Letter language
             </label>
             <div
               class="nhsuk-hint"

frontend/src/content/content.ts

@@ -459,7 +459,7 @@ const templateFormLetter = {
   templateNameHintText: 'This will not be visible to recipients.',
   templateTypeLabelText: 'Letter type',
   templateTypeHintText: 'Choose the type of letter template you are uploading',
-  templateLanguageLabelText: 'Additional language',
+  templateLanguageLabelText: 'Letter language',
   templateLanguageHintText: 'Choose the language of this letter template',
   templatePdfLabelText: 'Letter template PDF',
   templatePdfHintText: