You have some good corrections, so I don't have to add my two cents worth.  Here's my translation version :)

Yesterday, My mom accompanied me to buy a new watch that was to match my suit.

昨天与母親去買新錶来襯配我大礼服.Because my wrist is very thin, I decided to buy a watch that was thinner. 是因手腕幼細所以决定要个比較扁薄的手錶But most of these such watches were for women! 但大部份是屬於女式/裝. After choosing for half an hour, 選擇了半个钟头, I finally decided to buy one that was very very thin. 决定要這一條非常常扁薄的This watch has a two year international warranty, 这个有両年的国际保证/障 so if it breaks while I'm in London, 所以当我在倫敦, 如果有事的話, I can just fix it straight away. 就沒有问題去整修, I don't have to wait a year! 不須要等一年后帶回家修整It's really convenient!真方便啊!

Good luck to your London studies.  Perhaps you are still there when I come next January for a food trip!

买一套块新手表

昨天，妈妈陪我去买一套块陪可以搭配大衣的心新手表。由于我的风险很薄手腕很细，我确定买一套块小的手表。可是这样的标准款式大多苏数的手表都是给女生的！选了半个小时后，终于选了一套块很薄小的手表。这套它有两年的国际保修，如果我在伦敦时手表坏了，我可以直接把他修理好。不用等一年！真的很方便！

买一套新手表

昨天，妈妈陪我去买一套陪可以配上我的大衣的心新手表。由于我的风险很薄手腕很细，我确定决定买一套小细的手表。可是这样的标准大多苏的种手表大多数都是给女生的！选了半个小时之后，终于选了一套很薄细的手表。这套它有两年的国际保修，如果我在伦敦时手表坏了，我可以直接把他修理好。不用等一年！真的很方便！

买一套块新手表

昨天，妈妈陪我去买一块配（搭配）大衣的新手表。由于我的手腕很瘦，我决定买一块小的手表。可是这样的标准小的手表大多数(SHU)的手表都是给女生的为女生提供的！选了半个小时后，终于选了一块很薄的手表。这块表有两年的国际保修，如果我在伦敦时手表坏了，我可以马上把他修理好，不用等一年！真的很方便！