I'm quite surprised by the two super serious negative reviews of this episode. It sure was the least interesting and memorable Christmas episode we've had so far, but I enjoyed it nonetheless.
I liked the main story of Boyle in search of Captain Latvia. I have to say that knowing practically nothing about Latvian culture, none of the details about the country really made me lose interest of the episode itself. Although the story was not the most thrilling, and some parts were long and non-interesting, I liked seeing Charles getting involved in the case to make Nikolaj happy for his first Christmas. The jokes weren't the funniest, but it was a nice addition, as some really worked well.
The B-plot story with the singing competition wasn't the most gripping story either, but a few jokes were still worth it. It wasn't particularly good, but it was enjoyable overall.
All in all, I'd say this is the worst Christmas episode we've had so far. However, Charles going "Full Boyle" for his son's hapiness, and Jake's and Boyle's chemistry always work for me. Not desatrous, I guess i'd be willing to watch it on occasions.