lettisch
Latvian
Refers to Latvia, its inhabitants, its language, or its culture.
Er spricht fließend Lettisch und Russisch.
He speaks fluent Latvian and Russian.

Latvian
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Latvian
Refers to Latvia, its inhabitants, its language, or its culture.
Er spricht fließend Lettisch und Russisch.
He speaks fluent Latvian and Russian.

The adjective 'lettisch' (Latvian) is derived from the country 'Lettland' (Latvia). The ending '-isch' is typical for adjectives that describe origin or belonging to a country or region.
Die lettische Küste ist für ihre langen Sandstrände bekannt.
The Latvian coast is known for its long sandy beaches.
When 'lettisch' (Latvian) is used as the name of the language, it is capitalized and treated like a noun. It often appears with verbs such as 'sprechen' (to speak), 'lernen' (to learn), or 'verstehen' (to understand).
In der Schule habe ich ein Jahr lang Lettisch gelernt.
I studied Latvian for a year at school.
The adjective 'lettisch' (Latvian) describes things, but not people. To refer to a person from Latvia, use the nouns 'der Lette' (a Latvian man) and 'die Lettin' (a Latvian woman).
Meine neue Kollegin ist Lettin und kommt aus Riga.
My new colleague is Latvian and comes from Riga.
Baltic
'baltisch' and 'lettisch' can both refer to Latvia or to the Latvian language. 'lettisch' is specific and means Latvia, its people, culture, or language. 'baltisch' is more general and refers to the Baltic countries or the Baltic language family. The words can be confused because Latvia is a Baltic country and Latvian is a Baltic language. If you only mean Latvia, 'lettisch' is more natural. If you mean several countries or languages in this group, 'baltisch' fits.

Frau Klein lernt lettische Sätze, um mit einem Freund in Riga zu sprechen.
Ms. Klein is learning Latvian sentences so she can speak with a friend in Riga.
Nach der Nachricht las du einen Artikel über baltische Politik.
After the news, you read an article about Baltic politics.