Except for certain neighbourhoods in central Brussels and the outer edge of Antwerp the port and docks, Belgium is a safe country. Belgians are somewhat shy and introverted, but generally helpful towards strangers.
For those landing in Charleroi and Liège, those are the regions that boast the highest crime rates in Southern Belgium. But if you keep an eye on your belongings, and avoid wandering alone at night, nothing really serious is likely to happen to you.
Muslims and people of North African ancestry may experience mild resentment, a problem that is particularly acute in Brussels and Antwerp. The Burqa is illegal in public.
Marijuana laws are quite lenient - possession of up to 5 grams or one female plant is decriminalised but confiscated.
The emergency phone number in Belgium fire, police, paramedics is 112.
Belgium has three official languages: Dutch, French and German.
Virtually all official information will be only in the language of the region you are in. The same is true for public transport information. If you need to fill out government forms or submit documents in support of your application for government services e.g. a visa, resident permit, please take note that you need to give your responses in French, Dutch or German. The government does not recognise responses in any other language.
Please note that although Belgium has three official languages, that does not mean that all of them are official everywhere. In fact, language is one of the most politically sensitive/divisive issues in the country, and it may be considered offensive and unappreciated to speak the "wrong" language particularly French and Dutch at the "wrong" region. The only official language of Flanders is Dutch; Brussels has both Dutch and French as its official languages albeit the lingua franca is French. The only official language of Wallonia is French, except for the nine municipalities including the town of Eupen and its surroundings of the German-speaking Community.
A number of inhabitants of Wallonia, particularly the older generations, speak the Walloon language. This language, while not official, is recognized by the French Community of Belgium as an "indigenous regional language", together with a number of other Romance Champenois, Lorrain and Picard and Germanic Luxembourgian language varieties.
English is widely spoken by the younger generation of Dutch-speaking Belgians. In contrast, French-speaking Belgians rarely speak much English, though it is a much better and less bitter bet to use compared to Dutch. Consequently, one can get around Flanders without much problem speaking English, but if travelling around Wallonia, bringing a phrasebook or a smartphone translator app preferably with offline functions along is highly recommended.
Likewise, foreign TV programmes including news interviews to foreigners and films are subtitled to Dutch in Flanders except those catering to young children, and dubbed to French in Wallonia.
Belgians don't like to talk about their income or politics. You must also avoid asking people about their views on religion. Religion is considered a strictly private matter.
The Flanders-Wallonia question or dispute and the high number of separatist and extreme-right votes in Flanders are controversial topics and you must avoid asking people about their views on these as well. Keep any opinions or biases to yourself.
Do NOT tell the Walloons and most of the people of Brussels that they are French. Most Walloons, despite speaking French, are not and do not consider themselves French and dislike being associated with their neighbour France.
And for the same reason, do NOT tell the Flemish and also the people of Brussels that they are Dutch. Most Flemings, despite speaking Dutch Flemish, are not and do not consider themselves Dutch and dislike being associated with their neighbour the Netherlands.
Belgians in general are very proud of their comic book artists. The "Belgian school of comic books" is hailed as a national pride. In Belgium, comic books are valuable books printed with a hard cover. There are dozens of beautiful yet expensive merchandizing items, and the Belgians are fond of them. A plastic figurine of a comic book character or a special artwork of a hailed comic book artist would be a perfect gift for your Belgian friends and in-laws, for example.
Giving tips shows that you were content with the service given, but you are certainly not obliged to do so. It is sometimes done in bars and restaurants. Depending on the total, a tip of €0,50 to €2,50 is considered generous.
Have respect for the things that are made in Belgium or at least considered to be made. For example, you have to be respectful for the Belgian made fries.
Belgium has a modern telephone system with nationwide cellular telephone coverage, and multiple internet access points in all cities, free in most libraries. Also in multiple gas stations, NMBS/SNCB train stations and diners on the highways there is Wi-Fi available.
Many cafés offer free WiFi nowadays, but don't write it on the door for whatever reason...