i have created a forum from page in arabic langue when any user try to post any question it return 500:intrnal server error 

 how can i solve this even i search it didn't get the solve try to clear the browser cash and confirmed and validate the users but still get this error

and here the error and the trace

unknown locale 'ar_AA'
Traceback (most recent call last):
  File "/home/odoo/src/odoo/12.0/odoo/addons/base/models/qweb.py", line 346, in _compiled_fn
    return compiled(self, append, new, options, log)
  File "<template>", line 1, in template_website_forum_display_post_question_block_6463
  File "/home/odoo/src/odoo/12.0/odoo/addons/base/models/ir_qweb.py", line 387, in _get_field
    content = converter.record_to_html(record, field_name, field_options)
  File "/home/odoo/src/odoo/12.0/odoo/addons/base/models/ir_qweb_fields.py", line 126, in record_to_html
    return False if value is False else record.env[self._name].value_to_html(value, options=options)
  File "/home/odoo/src/odoo/12.0/odoo/addons/base/models/ir_qweb_fields.py", line 235, in value_to_html
    locale = babel.Locale.parse(lang.code)
  File "/usr/lib/python3/dist-packages/babel/core.py", line 330, in parse
    raise UnknownLocaleError(input_id)
babel.core.UnknownLocaleError: unknown locale 'ar_AA'

Error to render compiling AST
UnknownLocaleError: unknown locale 'ar_AA'
Template: website_forum.display_post_question_block
Path: /templates/t/div/small/span[4]
Node: <span t-field="question.write_date" t-options="{&quot;format&quot;:&quot;short&quot;}"/>

the trace 
Traceback (most recent call last):
  File "/home/odoo/src/odoo/12.0/odoo/addons/base/models/qweb.py", line 346, in _compiled_fn
    return compiled(self, append, new, options, log)
  File "<template>", line 1, in template_website_forum_display_post_question_block_6463
  File "/home/odoo/src/odoo/12.0/odoo/addons/base/models/ir_qweb.py", line 387, in _get_field
    content = converter.record_to_html(record, field_name, field_options)
  File "/home/odoo/src/odoo/12.0/odoo/addons/base/models/ir_qweb_fields.py", line 126, in record_to_html
    return False if value is False else record.env[self._name].value_to_html(value, options=options)
  File "/home/odoo/src/odoo/12.0/odoo/addons/base/models/ir_qweb_fields.py", line 235, in value_to_html
    locale = babel.Locale.parse(lang.code)
  File "/usr/lib/python3/dist-packages/babel/core.py", line 330, in parse
    raise UnknownLocaleError(input_id)
babel.core.UnknownLocaleError: unknown locale 'ar_AA'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/odoo/src/odoo/12.0/odoo/addons/base/models/ir_http.py", line 203, in _dispatch
    result = request.dispatch()
  File "/home/odoo/src/odoo/12.0/odoo/http.py", line 833, in dispatch
    r = self._call_function(**self.params)
  File "/home/odoo/src/odoo/12.0/odoo/http.py", line 344, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/home/odoo/src/odoo/12.0/odoo/service/model.py", line 97, in wrapper
    return f(dbname, *args, **kwargs)
  File "/home/odoo/src/odoo/12.0/odoo/http.py", line 340, in checked_call
    result.flatten()
  File "/home/odoo/src/odoo/12.0/odoo/http.py", line 1268, in flatten
    self.response.append(self.render())
  File "/home/odoo/src/odoo/12.0/odoo/http.py", line 1261, in render
    return env["ir.ui.view"].render_template(self.template, self.qcontext)
  File "/home/odoo/src/odoo/12.0/odoo/addons/base/models/ir_ui_view.py", line 1284, in render_template
    return self.browse(self.get_view_id(template)).render(values, engine)
  File "/home/odoo/src/odoo/12.0/addons/website/models/ir_ui_view.py", line 318, in render
    return super(View, self).render(values, engine=engine, minimal_qcontext=minimal_qcontext)
  File "/home/odoo/src/odoo/12.0/addons/web_editor/models/ir_ui_view.py", line 27, in render
    return super(IrUiView, self).render(values=values, engine=engine, minimal_qcontext=minimal_qcontext)
  File "/home/odoo/src/odoo/12.0/odoo/addons/base/models/ir_ui_view.py", line 1293, in render
    return self.env[engine].render(self.id, qcontext)
  File "/home/odoo/src/odoo/12.0/odoo/addons/base/models/ir_qweb.py", line 59, in render
    result = super(IrQWeb, self).render(id_or_xml_id, values=values, **context)
  File "/home/odoo/src/odoo/12.0/odoo/addons/base/models/qweb.py", line 274, in render
    self.compile(template, options)(self, body.append, values or {})
  File "/home/odoo/src/odoo/12.0/odoo/addons/base/models/qweb.py", line 348, in _compiled_fn
    raise e
  File "/home/odoo/src/odoo/12.0/odoo/addons/base/models/qweb.py", line 346, in _compiled_fn
    return compiled(self, append, new, options, log)
  File "<template>", line 1, in template_590_6443
  File "<template>", line 2, in body_call_content_6442
  File "<template>", line 37, in foreach_6439
  File "/home/odoo/src/odoo/12.0/odoo/addons/base/models/qweb.py", line 348, in _compiled_fn
    raise e
  File "/home/odoo/src/odoo/12.0/odoo/addons/base/models/qweb.py", line 346, in _compiled_fn
    return compiled(self, append, new, options, log)
  File "<template>", line 1, in template_website_forum_display_post_6449
  File "/home/odoo/src/odoo/12.0/odoo/addons/base/models/qweb.py", line 353, in _compiled_fn
    raise QWebException("Error to render compiling AST", e, path, node and etree.tostring(node[0], encoding='unicode'), name)
odoo.addons.base.models.qweb.QWebException: unknown locale 'ar_AA'
Traceback (most recent call last):
  File "/home/odoo/src/odoo/12.0/odoo/addons/base/models/qweb.py", line 346, in _compiled_fn
    return compiled(self, append, new, options, log)
  File "<template>", line 1, in template_website_forum_display_post_question_block_6463
  File "/home/odoo/src/odoo/12.0/odoo/addons/base/models/ir_qweb.py", line 387, in _get_field
    content = converter.record_to_html(record, field_name, field_options)
  File "/home/odoo/src/odoo/12.0/odoo/addons/base/models/ir_qweb_fields.py", line 126, in record_to_html
    return False if value is False else record.env[self._name].value_to_html(value, options=options)
  File "/home/odoo/src/odoo/12.0/odoo/addons/base/models/ir_qweb_fields.py", line 235, in value_to_html
    locale = babel.Locale.parse(lang.code)
  File "/usr/lib/python3/dist-packages/babel/core.py", line 330, in parse
    raise UnknownLocaleError(input_id)
babel.core.UnknownLocaleError: unknown locale 'ar_AA'

Error to render compiling AST
UnknownLocaleError: unknown locale 'ar_AA'
Template: website_forum.display_post_question_block
Path: /templates/t/div/small/span[4]
Node: <span t-field="question.write_date" t-options="{&quot;format&quot;:&quot;short&quot;}"/>