Relax. Two of mine didn't even speak at that age. My oldest didn't speak until age 2 1/2 and then it exploded like a damn breaking. The youngest did not have the magic 6 words at 18 months, the criteria in the U.S. for ST. She quickly learned
We didn't do that because ST is not available here in France. A child can only get it if they have a diagnosed condition (anything; premature, Downs, etc. but they need a reason for it). Very different philosophies!
My middle child started sentences at 20 months. I can't remember when she started speaking but she immediately separated the languages and never mixed.
I think personality played a big role. She was much more demanding than the other two. Can't tell others what you want if you can't say what it is lol!
Expect an inequality in the languages all their childhood. Sometimes it shifts, obviously if you move or even go on vacation. Their vocabulary will be better in one language over another just because of circumstances. I know horse riding vocab better in French lol! Never did it in my own country but it's big here where we live in France...
The pronunciation issue can continue through the school years. I know bi and trilingual children who quite frankly, funny sounding. My son had it too a bit in English. They're all fluent but it doesn't seem to set, almost as if uncooked... Hard to explain but these problems are not really problems. They're small issues which iron out over time and go away quickly with a bit more contact in that language.
I agree that some languages are easier than others for wee ones to say. I know French pronunciation is just easier than that in English. Same for Italian. I don't speak Russian but I understand that language is really hard for children to master. I only know a little Chinese but I know that those words are super-easy for babies to pick up and be understood.
So for instance, if there is a French-English bilingual in the U.S., they might pick up the languages faster than a French-English bilingual in France who has the same exposure. The one in the U.S. is getting more exposure to the harder-to-pronounce language. Make sense? Eventually, both should catch on with good contact in both languages.
May I point out that easier to pronounce may not necessarily mean that the language, overall, is easier to learn! Reading, writing and true mastery at an adult form might be harder. Chinese would be a good example. The writing is tough although the child learned to speak it well and early.
Exposure is key and one of the reasons this is hard to measure. If they child hears more Russian, they will speak that better than French, if they simply get less interaction in French overall.
Which brings me to sentences. Some languages can be expressed quite effectively without having to string the words together, like in English. In Chinese, they almost have to start putting words together immediately to speak at all. Many of their words are actually combinations. Meanwhile, French phrases are very easy and children use them almost immediately. "It is" "C'est" (said kind of like "say") is easy to learn. It took ages for my kids to say "It is like..." but in French, C'est... popped out almost immediately.
The grammar, is easier.
I think she's going great. I wouldn't sweat it!