I'm not familiar with the laws in those particular countries but many countries have neutrality laws that prohibit their citizens from serving in foreign militaries.

Canada still has the Foreign Enlistment Act[0].

"Any person who, being a Canadian national, within or outside Canada, voluntarily accepts or agrees to accept any commission or engagement in the armed forces of any foreign state at war with any friendly foreign state or, whether a Canadian national or not, within Canada, induces any other person to accept or agree to accept any commission or engagement in any such armed forces is guilty of an offence."[0]

There are questions on whether it applies (from 2022):

"Gordon Campbell, a military and criminal lawyer at Aubry Campbell MacLean, told CTVNews.ca that the Foreign Enlistment Act only applies to Canadians who formally enlist with a foreign military, and is not convinced it applies at all in this situation. He says it is not clear whether Russia is considered “friendly” or not to Canada in this situation, in the legal sense.

“I know others have speculated that Russia might not fall into that category,” he said in a phone interview. “I certainly would not be that certain on the point… We're not at war with Russia, but the word ‘friendly,’ is never defined in the act, so who knows.”

Campbell said those volunteering in Ukraine have to understand that they could be charged in Canada under Canada’s Criminal Code or under the Ukrainian legal system, depending on the circumstances.

“If you're a Canadian and you've travelled abroad and you engage in certain acts, then you can -- in theory -- be prosecuted inside Canada for those acts,” he said.

“Just because you're in the middle of a conflict doesn't exempt you from Canada's criminal laws.”[1]

[0] https://laws-lois.justice.gc.ca/eng/acts/F-28/FullText.html

[1] https://www.ctvnews.ca/canada/article/legal-questions-abound...

I was saying that Belgian, Canadian, and other countries could send their armies (and other armed forces) to fight the Russians in Ukraine.