Does the word “Motherland” always have to start with a capital letter?

See a translation

(1) Everybody has a motherland.
but
(2) The army defended The Motherland.

Beside the first words of sentences, the simple rule is: capitalise only all names. ("Name" here means a word or phrase that identifies one unique object or person - formally it would be called proper noun or a proper noun phrase.)

In sentence (1), "motherland" refers to a class of things.
In sentence (2) , "The Motherland" is used to name a single definite thing.

There are plenty of examples where people (and official style guides) break this rule, but you should be safe if you follow it.

Sometimes we capitalise adjectives derived from proper nouns, and some spell checkers may enforce this incorrectly. Probably we will all gradually conform to those programs, which always capitalise words like "french" because they cannot tell whether it is part of a proper noun phrase.

Correctly, following the rule stated above:
"We have meetings on mondays."
"We have a meeting on Monday." (names one specific day)
"If we have a meeting it must be a monday."
"He had a shakespearean tendency to misspell his name."
"Smallpox was common in Shakespearean England."
The man was french.
He spoke French.
"He and I are brothers." ("I" names the person speaking)