Q. May one make a siyum if certain parts of the masechta were learned mentally, but not verbalized?
May one make a siyum if he learned in English (Artscroll)?

A. Shulchan Aruch (O.H. 47: 4) rules that one may think in his mind thoughts of Torah without previously reciting the brocho on the Torah, presumably because he does not comply with the mitzva of learning Torah just by thinking or meditating Torah ideas. However, Mishna Berura (ibid. 7) quotes Hagra’s opinion that one should recite the brocho before engaging in Torah thoughts. (See Biur Halocho ibid) Poskim explain that there are various different mitzvos involved in Torah learning. One of them is “vehagisa” This book of the Torah shall not leave your mouth; you shall meditate therein day and night, (Yehoshua 1:8, see Sefer Charedim 11 – Tziunum Latorah 9).
See question 270 where we mention Horav Shlomo Miller’s Shlit”a opinion that you may perform a siyum on Torah learning done just by listening to a tape, as it is considered learning.
The Torah was also given and translated in all different languages (Sotah 32a), and you comply in any of them the mitzva.
Therefore, Horav Shlomo Miller’s Shlit’a opinion is that you can make a siyum when finishing a masechta that was learned in any language and even if it was learned only mentally, since it is considered learning Torah. (See similar ruling in Ohel Tomid quoting Horav Eliashiv zt’l).

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a