Ukraine should be completely left out of this question as it is not a member state or strongly tied to the EU. (In the context of the previous sentence, ‘strongly tied’ is implied to be a Switzerland or Norway-like relationship.) The question may still be asked about the Baltic states.
The European Union, however, does not regulate the individual member states’ language policies in any way. Whether or not member states designate an official national language, of if they designate one whether they designate more than one, and if they designate more than one whether they define a ranking between these languages is entirely up to the member states. Thus, the United Kingdom has no official de jure language at all, France has one official national language, Finland has two of which one is dominant in almost all areas of the country, Belgium has three of which two have an almost 50 % population share and so on. Germany has two designated regional minority languages, Finland has one, and Cyprus has two minority languages in addition to its two official languages. This principle is known as subsidarity and is one of the core principles of the EU: what can effectively be regulated at a national or regional level need not be regulated by the supranational EU.
In addition to what was said in the paragraph above, the EU 24 has official languages of which three are procedural languages and one (Irish Gaelic) is temporarily derogated due to the difficulty of finding translators. While it may seem as if these 24 languages were the de jure (or de facto where the former are lacking) national languages of the member states, that is in fact not the case. Turkish is an official language of Cyprus and Luxembourgish is the official language of Luxembourg yet neither are official EU languages.
Let me take a second to quickly break down your examples. Belgium was one of the founding states of the EU predecessor institutions, alongside the Netherlands, Luxembourg, Germany, France and Italy. Back then all three Belgian languages were already de jure or de facto languages of at least one further member state and two languages in general use in Luxembourg (German and French) were as well. There was no need for any special, specific regulation. When Finland joined the EU in 1995, it did so alongside Sweden and Austria. With Finnish being the vastly dominant language in Finland—as you note—it would obviously become one of the official EU languages. But so would Swedish because of Sweden where it is almost exclusively spoken.
On the other hand, when Ireland (and the UK) joined the EU’s predecessors in 1973, English obviously became official language but Irish was only granted the status of treaty language; meaning that it would be usable as a spoken language in the European Parliament and secondary legislation (i.e. not treaties) would not be translated into it. This was despite Irish enjoying the status of first official language by the constitution of Ireland. It was not until 2007 that the EU (more precisely: the foreign ministers) agreed to elevate Irish to an official language at the Republic’s request. Thus, in the European parliament where the languages are listed in a specific order (basically alphabetical by date of accession with caveats), Irish follows Greek (1981), Portugese, Spanish (both 1986), Finnish, Swedish (both 1995), Czech, Estonian, Latvian, Lithuanian, Hungarian, Maltese, Polish, Slovak and Slovene (all 2004).
As the case of Irish shows, becoming an official EU language is a matter of a member state requesting and the other member states accepting. As of now, no EU member state has a Russian majority, none have designated Russian as an official (co-) language and whether Russian is a regional minority language (such as Basque, Sami or Catalan) is largely irrelevant for EU official procedures.
Finally, I would like to point out that the Finnish and Belgian language situations have been a thing decades prior to the establishment of any EU predecessor. The Finnish language law practically dates back to its independence from Russia in 1917. So again, these are not issues where the EU said anything whatsoever.