Speaking specifically about the French Foreign Legion:
http://foreignlegion.info/
April 2024:
http://foreignlegion.info/2024/04/
May 2024:
http://foreignlegion.info/2024/05/
There are some former members of the FFL, now fighting alongside Ukraine troops inside Ukraine - some of them, as members of Ukraine’s international legion.
There are some Russians who were former members of the FFL, now fighting alongside Russian troops inside Ukraine.
But there is no regiment of the FFL, stationed in, nor heading to Ukraine at this time [May 5, 2024].
March 7, 2022:
https://foreignlegion.info/2022/03/07/french-foreign-legion-and-ukraine/
FFL HQ then assured Ukrainians who are members of the FFL, that they may take 2 weeks leave, in order to affect improvements regarding the security of their families.
Otherwise, there are some French Troops operating in non-combat roles in Ukraine.
Footnote: Members of the FFL, are from around the world - including from France.
That makes way more sense than an organized unit of the foreign legion. So essentially it’s mercenaries.