MIL-003: Step 2 - New flash cards

Review record RC-025 (Go, autonomous mode, not independent).

Task: MIL-003#1
Task: MIL-003#2
Task: MIL-003#3
Task: MIL-003#4
Task: MIL-003#5

src/flash_card/domain/value_objects/language_pair.py

@@ -0,0 +1,17 @@
+"""The two languages of a word list.
+
+@file language_pair.py
+@brief The LanguagePair value object.
+"""
+
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True, slots=True)
+class LanguagePair:
+    """The language codes of the front and the back of the cards."""
+
+    ## @brief Language code of the card front.
+    front_code: str
+    ## @brief Language code of the card back.
+    back_code: str