Christmas in Germany

差唔多聖誕，因為（所以）我今（次）寫關注（於）聖誕。

> 關注= concern, 關於＝ about

（^因為我喺香港），今年我無屋企人係身邊同我過聖誕，
因為我喺香港。所以我決定我會寫關注（於）聖誕喺德國（嘅聖誕）。
> You are pretty close in this sentence, just have to inverse the order:) 

Mostly, we use喺 when we say somebody in somewhere, e.g. 你喺香港; 嘅 can be the use of  sb's sth, e.g. 你嘅銀包（your wallet)

12月24日晚我哋喺德國過聖誕。我屋企人通常會去教會之後我哋一齊食晚餐。食完飯之後我地會交換禮物。我要有（準備）一個禮物比送比所有人，因此所有人從一個禮物送比我（佢地都會各自送一份禮物比我）。

> the first two i crossed is because their meaning have been repeated, like “完” has the meaning of finish something, you don't need to 之後afterward

準備=prepare, so i guess in this sentence you are trying to write that you have to prepare presents to each others and they will also do the same right? 
各自= respectively, separetly

25日同26日係公眾假期。喺25日我哋同好朋友去食飯。我哋次次去一樣（同一間）食肆。我哋食Gans mit Knödel und Rotkraut
> “間” is the measure word od restaurant, 同＝same

喺26日我哋屋企人通常唔出屋(門）。我哋捱夜同成日睇電視。
喺聖誕我想識喺香港你做咩

To be honest, this is my first time to correct article and actually you did really well, I hope my explanation will be understandable for you, feel free to ask me if you have any problems or you want to practice with it :) 

今年嘅聖誕節我未計劃好要做咩好，可能同朋友食飯或者周圍行下。雖然你今年冇辦法同屋企人過，不過我都希望你會鐘意喺香港過呢個節日啦！：）

Christmas in Germany

差唔多聖誕，因為我今寫關注聖誕。 差唔多到聖誕，所以我今次會寫關於聖誕節既野
今年我無屋企人係身邊同我過聖誕，
因為我喺香港。所以我決定我會寫關注聖誕喺德國。 關於德國既聖誕節

12月24日晚我哋喺德國過聖誕。我屋企人通常會去教會，之後我哋一齊食晚餐。食完飯之後我地會交換禮物。我要有一個禮物比送比所有人，因此所有人從一個禮物送比我。 所以我都會得到一份禮物

25日同26日號係公眾假期。喺25號我哋會同好朋友去食飯。我哋每次去一樣食肆。我哋食Gans mit Knödel und Rotkraut

喺26日我哋屋企人通常唔出屋會留係屋企。我哋捱夜同成日通宵睇電視。
喺聖誕我想識喺香港你做咩知你地會有咩做