Add lang-tag based task selection (lang-separation) (#863)

* lang based task selection

* use BCP 47 instead of ISO 639-1

* add Field(None, nullable=True)

* update migration script down_revision

backend/alembic/versions/2023_01_20_1650-160ac010efcc_use_en_instead_en_us_as_default_lang.py

@@ -0,0 +1,29 @@
+"""use 'en' instead 'en-US' as default lang
+
+Revision ID: 160ac010efcc
+Revises: 4f26fec4d204
+Create Date: 2023-01-20 16:50:00
+
+"""
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision = "160ac010efcc"
+down_revision = "4f26fec4d204"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_column("message", "lang")
+    op.add_column("message", sa.Column("lang", sa.String(length=32), server_default="en", nullable=False))
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_column("message", "lang")
+    op.add_column("message", sa.Column("lang", sa.VARCHAR(length=200), autoincrement=False, nullable=False))
+    # ### end Alembic commands ###