For Cyrillic is better UTF-2, and I have already done a converter from CP1251 to UCS-2, and from UTF-8 to UCS-2
Create all complete PDU for send the SMS is another question…
The function
http://forum.mikrotik.com/t/rextended-fragments-of-snippets/151033/1
help if you are forced to use GSM-7.
You can use directly the RouterOS function to send the message.
And can be easily adapted for own language, if transliteration is not good enough.
I apologize in advance for any mistranslation, it is not my intention to offend anyone.
For example with
/tool sms send lte1 phone-number=3939 message=“\CC\E8 \E2\F1\B3 \E1\F0\E0\F2\E8, \F5\EE\F7\E5\F8 \ED\E5 \F5\EE\F7\E5\F8.”
(where \CC\E8 \E2\F1\B3 \E1\F0\E0\F2\E8, \F5\EE\F7\E5\F8 \ED\E5 \F5\EE\F7\E5\F8. is Ми всі брати, хочеш не хочеш. on CP1251)
I do not receive anything (or better only " , .“) , but with
/tool sms send lte1 phone-number=3939 message=[$CP1251to7BITbyGHOST (”\CC\E8 \E2\F1\B3 \E1\F0\E0\F2\E8, \F5\EE\F7\E5\F8 \ED\E5 \F5\EE\F7\E5\F8.")]
I receive
Mi vsi brati, xochesh ne xochesh
(Probably is more precise “My vsi braty, khochesh ne khochesh.” and is easy to fix it changing on function и → y and х → kh for adapt own language…)