I would like to know what is the degree of knowledge of the Hebrew language one must have in order to daven in Hebrew. I understand that one can daven if one can read Hebrew, but is it enough if one knows all the letters, but is unsure of whether they are pronouncing the words correctly and reads them slowly? Also, what about the case when one reads certain words with Ashkenazi and others with Sephardi pronounciation?

Tefilah in Hebrew is Halachicly valid even if you don’t understand a word you say; according to some authorities it is even better. (Shulchan Aruch OC 101, 4; Yabia Omer 5 OC 12) As for the pronunciation, it is better to have one pronunciation and not to mix (Igrot Moshe OC 3, 5), Nonetheless, if this is only a temporary stage of learning and adjusting, that is perfectly normal and you shouldn't stress over the matter, Hashem can understand your prayers either way...