Added Vietnamese translations

[kevinpapst]

Description

Translated by @hoangduy1505 - see #1748

Can be tested at https://demo-branch.kimai.org/vi/timesheet/
=> eg. with user susan_super and password kitten

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I verified that my code applies to the guidelines (composer kimai:code-check)
• I updated the documentation (see here)
• I agree that this code is used in Kimai and will be published under the MIT license

[codecov]

Codecov Report

Merging #1758 into master will increase coverage by 0.28%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1758      +/-   ##
============================================
+ Coverage     92.14%   92.43%   +0.28%     
  Complexity     4855     4855              
============================================
  Files           449      449              
  Lines         15053    15047       -6     
============================================
+ Hits          13871    13908      +37     
+ Misses         1182     1139      -43     

Impacted Files	Coverage Δ	Complexity Δ
src/Ldap/LdapDriver.php	75.51% <0.00%> (+75.51%)	14.00% <0.00%> (ø%)