I do not have a strong preference between Reinier, Reginar, Raginar. I have not seen Raginare? But why would it be a German family Helmut? There is also a branch in England by the way, but that does not make it English.
1. They start in Lower Lotharingia, and all the profiles who are not in a later branch are in the "Belgian" area, straddling the Dutch-French language boundary. The first branches are in Hainaut, Namur and Louvain, with a proposal that the house of Loon is also a branch. As they are Franks we could try to reconstruct from an old spelling?
2. Anyway, they are quite early so modern German, French and Dutch are hardly the issue. Another approach is to ask what the main literature is written in, and that would arguably be French (for example Vanderkindere). But it is hard to be sure. Certainly Dutch, English, German and French are important in this case. It is a classic case of a family in the mixed up core of European history.
3. Yet another approach is to ask whether there is an international consensus about spelling. Probably because of French scholarship, I have seen that the French names of the dynasty Reinier, and Regnier are used in other languages such as Dutch and English, and I would guess also German? But as mentioned above, I think a lot of scholars also try for a kind of archaic spelling which comes closer to the original pronunciation, the same as they call early Gilbert Giselberts or even Giselbrechts (but not modern Dutch Gijzelbrechts).
Short version: I am guessing Reginar and Raginar are both simple and recognizable, but honestly I have been seeing more spellings with Re than Ra?. Raginare surprised me. Reinier and Regnier are very common but maybe too "modern French" but they are common in the literature.
I just browsed some of the primary literature in Vanderkindere and I see that the Latin also used both Re and Ra (typically Re(g)in(h)erus or Ra(g)in(h)erus).
FWIW, although people keep saying it looks Norse Ragnar, I was recently working on Clovis I and his noble relative was named Raginchar, a king living in this same Austrasian March. Looks like the same name. I would say that the Re spellings are coming from the effect of a Dutch style "g" sound which can work like an English "y". Could Ragnar be a Frankish name originally?