English is flexible, Alexandre. Before answering your question, one more way to express what was originally asked:
Aline said that she would start studying English and Russian at the same time, but I don´t really think she means it.
So, you can say something in several ways, that includes the use of "trust" and "believe". Anyway, the way one put it can make a whale of a difference.
For instance: Aline said that she would start studying English and Russian at the same time, but I don´t really she will.
Here it could be that "eu não sinto firmeza" in the sense that she probably won´t do what she says, that is. One feels that she doesn´t mean what she says"
It could be that she is a full-fledged liar (she frequently lies and certainly it won´t be any different this time). In that situation I just don´t trust what she says, ergo - I don´t trust what she´s saying right now.
Summing up, "não sentir firmeza" could mean "não acreditar/não confiar" but it could also be "não acreditar muito/achar que a pessoa não vai fazer ou não vai conseguir fazer algo".
The "...I don´t think she meant/means it." could cover both senses (not at the same time, of course). The context would be what would express which one you wish to express.