direct translation is お昼ごはんを食べに行きませんか。 ohiru gohan wo tabeni ikimasenka? so if u say like that friendly, u can say ”おひる一緒にどうですか”　ohiru isshoni doudesuka? would you mind if u go out lunch together? だと、自然ですね。 then it's more natural. i think

I'm just a beginner, so this may not be completely correct.. but.. いっしょにレストランでたべませんか issho ni (together) resutoran (restaurant) de (at) tabe masen ka (would you like to eat) so.. issho ni resutoran de tabe masen ka いっしょにレストランでたべませんか