I’ve been trying to find a translation for the phrase, but I m not very sure it sounds natural in Japanese. It also has to fit quite a tight space, so I can't really make the text too long. If anyone can help with some advice, it would be really appreciated.
This is what I have so far. 西部で一番人気タイトルは日本へ！
「西部{せいぶ}で一番人気{いちばんにんき}タイトルは日本へ！」
The two parts that native Japanese-speakers will instantly find unnatural-sounding are the word 「西部」 and the particle「は」.
「西部」 does not mean "the West" in the sense of the "Occident". It just means the western part of a town, region, country, etc. For the U.S., for instance, 「西部」 means states such as California.
The word you are looking for would be 「欧米{おうべい}」 or 「西洋{せいよう}」 and for advertisement, the former would by far be the most natural word choice.
The particle would definitely be 「が」 here, (but I myself might not be using a particle). For advertisement copies, particles can seriously get in the way of impactfulness.
My recommendations:
「欧米一番人気のタイトルが（ついに）日本上陸{にほんじょうりく}！」
「欧米一番人気のタイトル,（ついに）日本上陸！」
「欧米一番人気タイトル,（ついに）日本上陸！」
「ついに」 ("finally") can easily be dropped if the space is very tight.
The grammar "rules" that apply to advertisement copies are very similar to those that apply to headlines for articles that are discussed in this Q&A.
