Implement proper locale check (#60)

# Implement proper locale check

## ♻️ Current situation & Problem
Closes #20 


## ⚙️ Release Notes 
* Implement proper locale check

### Code of Conduct & Contributing Guidelines 

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of
Conduct](https://github.com/StanfordBDHG/.github/blob/main/CODE_OF_CONDUCT.md)
and [Contributing
Guidelines](https://github.com/StanfordBDHG/.github/blob/main/CONTRIBUTING.md):
- [x] I agree to follow the [Code of
Conduct](https://github.com/StanfordBDHG/.github/blob/main/CODE_OF_CONDUCT.md)
and [Contributing
Guidelines](https://github.com/StanfordBDHG/.github/blob/main/CONTRIBUTING.md).

modules/firebase/localizedText.test.ts

@@ -0,0 +1,52 @@
+//
+// This source file is part of the Stanford Biodesign Digital Health ENGAGE-HF open-source project
+//
+// SPDX-FileCopyrightText: 2023 Stanford University and the project authors (see CONTRIBUTORS.md)
+//
+// SPDX-License-Identifier: MIT
+//
+import { createLocalizationHelpers } from '@/modules/firebase/localizedText'
+
+const { parseLocalizedText, parseNilLocalizedText } =
+  createLocalizationHelpers('pl-PL')
+
+describe('parseLocalizedText', () => {
+  const plPL = 'pl-PL'
+  const pl = 'pl'
+  const en = 'en'
+  const de = 'de'
+
+  it('supports plain string', () => {
+    expect(parseLocalizedText(en)).toBe(en)
+  })
+
+  describe('localized text object', () => {
+    it('fallbacks to empty string when no localizations', () => {
+      expect(parseLocalizedText({})).toBe('')
+    })
+
+    it('fallbacks to "en" if no matching client languages', () => {
+      expect(parseLocalizedText({ de, en })).toBe(en)
+    })
+
+    it('fallbacks to any translation if there is no "en" available', () => {
+      expect(parseLocalizedText({ de })).toBe(de)
+    })
+
+    it('resolves ISO-639-1 standard', () => {
+      expect(parseLocalizedText({ pl, en })).toBe(pl)
+    })
+
+    it('prioritizes ISO-639-2 if exists', () => {
+      expect(parseLocalizedText({ 'pl-PL': plPL, pl, en })).toBe(plPL)
+    })
+  })
+})
+
+describe('parseNilLocalizedText', () => {
+  it('supports nil values', () => {
+    expect(parseNilLocalizedText(null)).toBe(null)
+    expect(parseNilLocalizedText(undefined)).toBe(null)
+    expect(parseNilLocalizedText('')).toBe('')
+  })
+})

modules/firebase/localizedText.ts

@@ -9,11 +9,40 @@ import { isNil } from 'es-toolkit'
 import { isObject } from 'lodash'
 import { type LocalizedText } from '@/modules/firebase/models'
 import { type Nil } from '@/packages/design-system/src/utils/misc'
+import { getNavigatorLanguage } from '@/packages/design-system/src/utils/navigator'
 
-const locale = 'en'
+export const createLocalizationHelpers = (locale: string) => {
+  const possibleLocalesToCheck = [
+    locale,
+    locale.includes('-') ? locale.split('-').at(0) : null,
+    'en',
+    'en-US',
+  ].filter(Boolean)
 
-export const parseLocalizedText = (text: LocalizedText) =>
-  isObject(text) ? text[locale] : text
+  const parseLocalizedText = (localizedText: LocalizedText) => {
+    if (!isObject(localizedText)) return localizedText
+    for (const locale of possibleLocalesToCheck) {
+      const text = localizedText[locale] as string | undefined
+      if (text !== undefined) {
+        return text
+      }
+    }
+    const fallbackAnyText = Object.values(localizedText).at(0)
+    if (fallbackAnyText === undefined) {
+      console.error('localized text has no localisations provided')
+      return ''
+    }
+    return fallbackAnyText
+  }
 
-export const parseNilLocalizedText = (text: Nil<LocalizedText>) =>
-  isNil(text) ? null : parseLocalizedText(text)
+  const parseNilLocalizedText = (localizedText: Nil<LocalizedText>) =>
+    isNil(localizedText) ? null : parseLocalizedText(localizedText)
+
+  return { parseLocalizedText, parseNilLocalizedText }
+}
+
+const { parseLocalizedText, parseNilLocalizedText } = createLocalizationHelpers(
+  getNavigatorLanguage(),
+)
+
+export { parseLocalizedText, parseNilLocalizedText }

packages/design-system/src/utils/navigator.ts

@@ -0,0 +1,16 @@
+//
+// This source file is part of the Stanford Biodesign Digital Health ENGAGE-HF open-source project
+//
+// SPDX-FileCopyrightText: 2023 Stanford University and the project authors (see CONTRIBUTORS.md)
+//
+// SPDX-License-Identifier: MIT
+//
+/* eslint-disable @typescript-eslint/no-unnecessary-condition */
+/**
+ * Gets user locale, providing en-US as fallback
+ * */
+export const getNavigatorLanguage = () => {
+  if (typeof window === 'undefined') return 'en-US' // Fallback for SSR
+  return navigator.languages.at(0) ?? navigator.language ?? 'en-US'
+}
+/* eslint-enable @typescript-eslint/no-unnecessary-condition */