The files lang_en_US.properties and lang_ru_RU.properties are maintained by the Legacy Launcher-developers.
All new strings are added to these files.
You should use one of these files as the source for your translation.
Edit one of the existing lang_*_.properties-files or create a new one.
Creation of a translation file for your language as a little bash script:
# change to your desired language code
lang=de_DE
while read LINE; do
echo "$LINE" | grep -q = && (
KEY=$(echo "$LINE" | cut -d = -f1 | sed 's/\([^#[:alnum:]]\)/\\\1/g')
grep "^${KEY}=" lang_${lang}.properties || echo "#TRANSLATE $LINE"
) || echo "$LINE"
done < lang_en_US.properties > translateme.txtThis creates a file translateme.txt with commented lines starting with #TRANSLATE, which are not in the current target language file. Edit it by your desire and move it to your targeted language file.