import re
s = "abc def déf äëü المزيد 한글 - 1 2 3 4 5"
print( re.sub(r"\b[a-zA-Z]*[^\W\d_a-zA-Z][^\W\d_]*\b", "<FOREIGN>", s) )
aW1wb3J0IHJlCnMgPSAiYWJjIGRlZiBkw6lmIMOkw6vDvCDYp9mE2YXYstmK2K8g7ZWc6riAICAtIDEgMiAzIDQgNSIKcHJpbnQoIHJlLnN1YihyIlxiW2EtekEtWl0qW15cV1xkX2EtekEtWl1bXlxXXGRfXSpcYiIsICI8Rk9SRUlHTj4iLCBzKSAp