i18n: add i18/l10n customisation

- docu: translation as invenioRDM feature

Refs inveniosoftware/invenio-i18n#126

docs/customize/i18n-and-l10n.md

@@ -0,0 +1,159 @@
+# Internationalisation (i18n) and Localisation (l10n)
+
+InvenioRDM, by default, comes with an English user interface. You can turn on
+additional languages and change the default one in your InvenioRDM instance.
+
+## Add Another Language
+
+### Step 1 - Adjust your invenio.cfg file
+
+In your `my-site/invenio.cfg` change:
+
+``` diff
+# Invenio-I18N
+# ============
+# See https://invenio-i18n.readthedocs.io/en/latest/configuration.html
+
+# Other supported languages (do not include BABEL_DEFAULT_LOCALE in list).
+I18N_LANGUAGES = [
+-   # ('de', _('German')),
+    # ('tr', _('Turkish')),
+]
+
+# Invenio-I18N
+# ============
+# See https://invenio-i18n.readthedocs.io/en/latest/configuration.html
+
+# Other supported languages (do not include BABEL_DEFAULT_LOCALE in list).
+I18N_LANGUAGES = [
++   ('de', _('German')),
+    # ('tr', _('Turkish')),
+]
+```
+
+!!! info "Letter Code"
+    The following letter code structure is supported: two letters like, `de` or
+    `tr` and four letters, like `de_AT`, `de_DE` or `zh_CN`.
+
+!!! info "Default and Additional Languages"
+    Make sure **not** to include the default language (`BABEL_DEFAULT_LOCALE`) in
+    the list of addtional languages (`I18N_LANGUAGES`). Both variables are set
+    within `my-site/invenio.cfg`.
+
+### Step 2 - Start or Re-run the server
+
+If you have a running local install:
+
+```shell
+<Ctrl+C>
+invenio-cli run
+```
+
+!!! info "Re-run when invenio.cfg changes"
+    All changes to `invenio.cfg` **MUST** be accompanied by a restart like the
+    above to be picked up. This only restarts the server; it does not destroy
+    any data.
+
+Otherwise, follow the [Build, setup and
+run](../install/build-setup-run.md)-instructions.
+
+### Step 3 - Change the Language in your Application
+
+In your browser, open your invenioRDM application (local install:
+[https://127.0.0.1:5000](https://127.0.0.1:5000); container install:
+[https://127.0.0.1](https://127.0.0.1)) or refresh the open application page.
+
+In the footer you find a drop down menu, where you can switch between different
+languages.
+
+![Language dropdown menu in footer](./img/i18n-language-dropdown.png)
+
+!!! warning "That evil cache"
+    If you do not see the dropdown-menu in the footer, check in an incognito
+    window; the browser might shows you a cached page.
+
+!!! info "Available Languages"
+    InvenioRDM provides several languages, for example, Arabic (`ar`), Chinese
+    (`zh_CN`), German (`de`), and Spanish (`es`). If your letter code does not
+    work, test with German (`de`). If you want your language to be included,
+    see: [Contribution](../contribute/translators-guide.md)
+
+## Change the Default Language
+
+### Step 1 - Adjust your invenio.cfg file
+
+In your `my-site/invenio.cfg` change:
+
+``` diff
+# Flask-Babel
+# ===========
+# See https://flask-babel.tkte.ch/#configuration
+
+# Default locale (language)
+- BABEL_DEFAULT_LOCALE = 'en'
+
+# Flask-Babel
+# ===========
+# See https://flask-babel.tkte.ch/#configuration
+
+# Default locale (language)
++ BABEL_DEFAULT_LOCALE = 'de'
+```
+
+!!! info "Letter Code"
+    The following letter code structure is supported: two letters like, `de` or
+    `tr` and four letters, like `de_AT`, `de_DE` or `zh_CN`.
+
+!!! info "Default and Additional Languages"
+    Make sure **not** to include the default language (`BABEL_DEFAULT_LOCALE`) in
+    the list of addtional languages (`I18N_LANGUAGES`). Both variables are set
+    within `my-site/invenio.cfg`.
+
+### Step 2 - Start your instance
+
+If you have a running local install:
+
+```shell
+<Ctrl+C>
+invenio-cli run
+```
+
+!!! info "Re-run when invenio.cfg changes"
+    All changes to `invenio.cfg` **MUST** be accompanied by a restart like the
+    above to be picked up. This only restarts the server; it does not destroy
+    any data.
+
+Otherwise, follow the [Build, setup and
+run](../install/build-setup-run.md)-instructions.
+
+### Step 3 - See your Language in the Application
+
+In your browser, open your invenioRDM application (local install:
+[https://127.0.0.1:5000](https://127.0.0.1:5000); container install:
+[https://127.0.0.1](https://127.0.0.1)) or refresh the open application page.
+
+InvenioRDM's user interface should now, by default, use the language you set
+via the `invenio.cfg`-file.
+
+!!! warning "That evil cache"
+    If your user interface doesn't talk to you in the expected language, check
+    in an incognito window; the browser might shows you a cached page.
+
+!!! info "Available Languages"
+    InvenioRDM provides several languages, for example, Arabic (`ar`), Chinese
+    (`zh_CN`), German (`de`), and Spanish (`es`). If your letter code does not
+    work, test it with German. If you want your language to be included, see:
+    [Contribution](../contribute/translators-guide.md)
+
+## Troubleshooting
+
+- There are English texts showing up in your user interface: If the application
+  cannot find a translation, it will fall back to English. You can help filling
+  these gaps by [becoming a translator](../contribute/translators-guide.md).
+- Strings appear in many places in your instance and different techniques are
+  involved to handle them. We are working on documentation for translations of
+    - (customized) vocabularies
+    - static pages
+    - (customized) emails
+- Ask for knowledge in [Discord `#rdm-translations`](https://discord.gg/Ya7qSG43Br).
+

docs/customize/index.md

@@ -34,3 +34,4 @@ In the following sections, we cover customization opportunities that InvenioRDM
     - [Names](vocabularies/names.md)
     - [Subjects](vocabularies/subjects.md)
     - [Users](vocabularies/users.md)
+- [Internationalisation (i18n) and Localisation (l10n)](i18n-and-l10n.md)

mkdocs.yml

@@ -79,6 +79,7 @@ nav:
       - Upload Limits: "customize/upload_limits.md"
       - Metadata-only records: "customize/metadata_only.md"
       - Logging: "customize/Logging.md"
+      - Internationalisation (i18n) and Localisation (l10n): "customize/i18n-and-l10n.md"
   - Develop:
       - Overview: "develop/index.md"
       - Getting started: