If one is not proficient in Hebrew, can he fulfil his obligation of ‘shnayim mikra v’echad targum’ by reading a translation and commentaries in English (say, from Artscroll).
Secondly, if one does not have a shul to daven at on Shabbat, is there any inyan to read the pesukim out loud even while just by himself, or does reading ‘mentally’ suffice?
Answer:
Reading the English is equivalent to reading the targum according to some authorities (though most require the reading of the actual targum–see Tosafos, Berachos 8b, and Rosh no. 8), but not the mikra, which can only be read in Hebrew.
If it is hard, however, then you should read the English translation, which will be the mikra, and a commentary (such as a translation of Rashi) for the targum. This isn’t the precise equivalent to the Talmudic obligation, but it comes close.
If you don’t have a shul where to daven, reading mentally suffices. However, reading out loud is a worthy practice as a zecher to reading in shul, although there is no ‘obligation’ to do this.
